Support `decode_responses` for string ops

[romanlevin]

Addresses #97

---

This change is 

[romanlevin]

@srikalyan Could this and/or #119 ever be merged?

[linuxmaniac]

@romanlevin It seems something is still missing, if you use hgetall() with decode_responses = True you will get bytes not strings:

--- a/mockredis/tests/test_hash.py
+++ b/mockredis/tests/test_hash.py
@@ -25,6 +25,13 @@ class TestRedisHash(object):
         self.redis.hset(hashkey, "key", "value")
         eq_({b"key": b"value"}, self.redis.hgetall(hashkey))

+    def test_hgetall_decode_responses(self):
+        self.redis.decode_responses = True
+        hashkey = "hash"
+        eq_({}, self.redis.hgetall(hashkey))
+        self.redis.hset(hashkey, "key", "value")
+        eq_({"key": "value"}, self.redis.hgetall(hashkey))
+
     def test_hdel(self):
         hashkey = "hash"
         self.redis.hmset(hashkey, {1: 1, 2: 2, 3: 3})

tox -e py35
GLOB sdist-make: /home/vseva/projects/mockredis/setup.py
py35 create: /home/vseva/projects/mockredis/.tox/py35
py35 inst: /home/vseva/projects/mockredis/.tox/dist/mockredispy-2.9.3.zip
py35 installed: mockredispy==2.9.3,pkg-resources==0.0.0
py35 runtests: PYTHONHASHSEED='3169646320'
py35 runtests: commands[0] | python setup.py nosetests
running nosetests
running egg_info
writing entry points to mockredispy.egg-info/entry_points.txt
writing mockredispy.egg-info/PKG-INFO
writing dependency_links to mockredispy.egg-info/dependency_links.txt
writing requirements to mockredispy.egg-info/requires.txt
writing top-level names to mockredispy.egg-info/top_level.txt
reading manifest file 'mockredispy.egg-info/SOURCES.txt'
writing manifest file 'mockredispy.egg-info/SOURCES.txt'
Searching for redis>=2.9.0
Reading https://pypi.python.org/simple/redis/
Downloading https://pypi.python.org/packages/09/8d/6d34b75326bf96d4139a2ddd8e74b80840f800a0a79f9294399e212cb9a7/redis-2.10.6.tar.gz#md5=048348d8cfe0b5d0bba2f4d835005c3b
Best match: redis 2.10.6
Processing redis-2.10.6.tar.gz
Writing /tmp/easy_install-7ihbyknp/redis-2.10.6/setup.cfg
Running redis-2.10.6/setup.py -q bdist_egg --dist-dir /tmp/easy_install-7ihbyknp/redis-2.10.6/egg-dist-tmp-aoz8fgls
warning: no previously-included files found matching '__pycache__'
warning: no previously-included files matching '*.pyc' found under directory 'tests'
zip_safe flag not set; analyzing archive contents...
Moving redis-2.10.6-py3.5.egg to /home/vseva/projects/mockredis/.eggs

Installed /home/vseva/projects/mockredis/.eggs/redis-2.10.6-py3.5.egg
.........F.....................................................................................SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS....................................................................................................
======================================================================
FAIL: mockredis.tests.test_hash.TestRedisHash.test_hgetall_decode_responses
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vseva/projects/mockredis/.eggs/nose-1.3.7-py3.5.egg/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/vseva/projects/mockredis/mockredis/tests/test_hash.py", line 33, in test_hgetall_decode_responses
    eq_({"key": "value"}, self.redis.hgetall(hashkey))
AssertionError: {'key': 'value'} != {b'key': b'value'}

----------------------------------------------------------------------
Ran 228 tests in 3.107s

FAILED (SKIP=33, failures=1)
ERROR: InvocationError: '/home/vseva/projects/mockredis/.tox/py35/bin/python setup.py nosetests'
________________________________________________________________________________________________________________________________________ summary _________________________________________________________________________________________________________________________________________
ERROR:   py35: commands failed