Revamp MediaHelper

As described in https://github.com/bmcclure/CakePHP-Media-Plugin/issues/49#issuecomment-28238733, this patch switches back to extending `AppHelper`, and ditches manual tag generation and overriding `Helper::_parseAttributes()` in favor of using `HtmlHelper::useTag()` and utilizing config files (as suggested by @gBokiau in https://github.com/bmcclure/CakePHP-Media-Plugin/pull/50#issuecomment-28200398) to configure `HtmlHelper` with proper tags and minimizable attributes.

It should be backwards compatible and resolve #50.