Python3 integration

[charly24]

refs: https://github.com/RyanBalfanz/django-sendgrid/issues/73 My project is Python3. So I fixed some compatibility below.

---

dict.iteritems to dict.items

import urllib2 to

try:
    import urllib.request as urllib2
except ImportError:
    import urllib2

import simplejson as json to

try:
    import simplejson as json
except ImportError:
    import json

isinstance(categoryData, basestring): to

from six import string_types
isinstance(categoryData, string_types)

import StringIO to

try:
    import StringIO
except ImportError:
    from io import StringIO

remove unused import httplib

remove some . and spaces.

https://github.com/RyanBalfanz/django-sendgrid/pull/76/files?w= (diff without spaces)