Added optional support for Zepto in addition to jQuery

This PR addresses [issue #3340](https://github.com/angular/angular.js/issues/3340), adding support for Zepto as well as jQuery.

This modification will opt for jQuery over Zepto whenever both are present.

Note:  This also requires a modification to karma-ng-scenario, which I will submit a PR for to pair with this PR.

Warning:  There may be tests/code that I did not write that should be written to account for the differences between jQuery, Zepto, and Angular, so it is possible that this is not complete - all of the current tests pass though, which otherwise break badly.

The documentation would need to be rewritten to note this feature as well.