Add support for browser-side caching of webapi resources.

WebAPIResource now has a few new properties and methods for handling
browser-side caching. Resources can handle Last Modified timestamps or
ETags, and generate them either based on fields or through custom functions.

This can speed up access to resources, improving performance both
client-side and server-side.

Tested with the accompanying Review Board change, both through unit tests
and checking responses on resources using Firebug. This tested auto-generated
ETags, Last Modified timestamps, and custom functions.