Support customizable prefix for getSkinName

* Derived from https://github.com/yui/yui3/pull/327, submitted by @aversini

* Submitting to dev-3.x, since it's new feature support for a core component and so that it gets some preview coverage since I'm not monitoring YUI day-to-day.

* All widget unit tests pass. All slider unit tests pass also, as do slider example tests [ slider is the only thing which uses getSkinName currently AFAIK ].

* Main src file change: https://github.com/yui/yui3/pull/397/files#diff-5 (wasn't sure if we still commit build files or not so they're in the pull request)