Add a use_cassette decorator

[sigmavirus24]

Related to #61

---

TODO:

• [x] Test with unittest's TestCase classes
• [x] Test with py.test
• [x] Test the decorator
• [x] Document the decorator

[sigmavirus24]

import unittest

from betamax.decorator import use_cassette

@use_cassette('test-use-cassette-decorator', cassette_library_dir='.')
def test(session):
    session.get('https://httpbin.org/get')

class TestExample(unittest.TestCase):
    @use_cassette('test-unittest', cassette_library_dir='.')
    def test_stuff(self, session):
        session.get('https://httpbin.org/get')

if __name__ == '__main__':
    test()
    unittest.main()

Is what I've been using to test this.

[sigmavirus24]

Now that the docs rewrite has been merged, I feel like we can definitely write good documentation around this and fit it into the new documentation appropriately.