This change speeds up the ClientBundle and CssResource generators.

- Use the compiler's ResourceOracle instead of the ClassLoader to find resources
- Add a caching mechanism to ClientBundle's ResourceContext to allow ResourceGenerators to cache data between invocations.
- Update CssResourceGenerator to use the cache for obfuscated type names.

The cache in ResourceContext will be discarded whenever the TypeOracle changes or its reload count increases.

Patch by: bobv
Review by: rice

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5566 8db76d5a-ed1c-0410-87a9-c151d255dfc7