Add DjbletsJSONEncoder for lazy string and datetime serialization.

We had two places where datetimes were serialized to chop off the
extra precision. Turns out we needed a third, for json_dumps. These
other places also didn't know how to handle lazy strings, which only
json_dumps knew how to do.

There's now a DjbletsJSONEncoder that can take on these
responsibilities. BasicAPIEncoder, ResourceAPIEncoder, and json_dumps
now make use of this instead of DjangoJSONEncoder.
Switch over to django's bundled six library.

Now that we've moved to Django 1.6, we no longer need to ship our own copy of
six. This changes all the imports and removes the shipped copy.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5496/