Make kew compile with the Closure Compiler.

- all(): Remove duplicate var counter
- Specify context type as * (ANY) instead of Object
- Add optional context to onSuccess, onFail
- underscore methods are @private
- make Error not nullable for onFail callbacks
kew: Make data * (not Object); context parameter now is *

Fixes nicks's code review comments.
Merge pull request #22 from evanj/master

Make kew compile with the Closure Compiler.