* added skip_if parameter and updated docs for it
added django settings option to disable rate limiting across the board and updated docs
Minor docs tweak
Reverse RATELIMIT_DISABLE_ALL.
Raise Ratelimited exception.

Instead of returning an HttpResponseForbidden, raise a Ratelimited
exception (a subclass of PermissionDenied) so middleware can handle it.

Adds optional middleware to capture the exception and use a view based
on a setting.
Tests!

- Add basic tests for @ratelimit.
- Add requirements for tests.
- Add Travis-CI file.
Real docs.