JSONSerializer compatibility for Python 3 bytes

* Update tests `FakeResponse` for Python 3 using `six`
* Update `JSONSerializer` to decode utf-8 before it loads it
* Update .gitignore
* Some too long lines in Django tests

Related issue: #14 
Even if that's the case, there's no need to commit it and send it as part of an unrelated pull request. It's poor form and distracting to change the style of code (even in tests) during a bug fix.