S3 Upload Signature API

Django'ed S3 upload signer API. You can access it like this:

    http://localhost:8000/api/s3_upload_signature/?s3_object_name=foo.bar&s3_object_type=text/plain

You can also use POST if you prefer.

No auth implemented, so beware.

Errors are spat out as JSON (with correct HTTP status codes, naturally).

I also cleaned up a few things that were bugging me, and things that were necessary to get the app running happily.