Fix aliased text when animating login #bubble

Previous discussion here: https://github.com/woothemes/FlexSlider/issues/222, example: http://www.youtube.com/watch?v=9Woaz-cKPCE&hd=1

Seems to be a bug in WebKit where animation causes font-smoothing (even within a sub-element) to affect the entire containing element.

Fixes #338