Python3 integration

[charly24]

refs: #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)