Add support for annotations

[gkossakowski]

Jribble backend skips annotations at the moment. GWT uses annotations for various purposes so we need to add suport for them.

This issue depends on scalagwt/scalagwt-jribble#1.

[stephenh]

Note that Aaron has been kicking the tires of UiBinder support and, while it's not working, hasn't run into any "annotation cannot be found errors".

I think we may be fine as is because GWT populates the TypeOracle's annotations from the bytecode. AFAICT, the GWT AST nodes like JMethod don't have a place to put annotations anyway.

If this sounds right, I think we can close this issue. Let me know if I'm missing something.