feat(noConflict): restore previous namespace

Angular needs to provide a way to restore the previously defined `angular` global namespace to prevent conflict with existing scripts. This is especially important if there are multiple versions of angular loaded onto the same page. If angular is used in a thirdparty javascript environment it should provide a way to clean up the global after loading. 

This feature is similiar to the jQuery [noConflict](http://api.jquery.com/jQuery.noConflict/) function.