fix(noConflict): resolve global JSONP conflicts and window.angular cannibalization

Eliminate `window.angular.callbacks` and use random callback handles on `window` to eliminate multiple angular version conflicts.

Also sure to start with an empty `window.angular` object rather than polluting existing `window.angular`. Prior, even though a reference to `window.angular` was kept in a local variable `_angular`, angular would load itself onto `window.angular` anyway polluting `_angular` in the process.

This is a follow-up to this rejected pull request: https://github.com/angular/angular.js/pull/2731 and this thread on the mailing list: https://groups.google.com/forum/?fromgroups#!searchin/angular/noConflict/angular/G8xPyD1F8d0/7m96EbK2l1oJ

Thanks!
Mike