Add support for per-resource mimetypes in the API.

Resources in the API are now returned with per-resource mimetypes. These
allow a client to identify the resource and make an intelligent decision
about what that payload represents without having to hard-code anything
about the location of the resource. An example would be client bindings
that could not only generically handle any resources but could also
provide utility functions for specific ones.

Unit tests have been updated to verify each and every resource we
provide.

This change will be made available for 1.5 and 1.6, so that we could
take advantage of this in RBTools.

Unit tests pass.

Verified that post-review still works as expected.

Checked to make sure the resources were returning sane mimetypes. There's
a chance we'll want to tweak some (we have different mimetypes for a resource
living in different locations, which I think is fine, but in practice they
may be the same), but everything appears to be what we'd want, and are
returning the correct results.