jquery.tipsy: Merge some upstream changes

https://github.com/jaz303/tipsy/tree/2d916c6

4b3e29f: Remove superfluous slash
23994f5: tipsy object proxy method tolerates null object
2d916c6: Always ensure native tooltip is restored

Change-Id: Id87ba461265730a56a3212c003b8321bd53a6096
Merge "jquery.tipsy: Merge some upstream changes"