[SPARK-2384] Add tooltips to UI.

This patch adds tooltips to clarify some points of confusion in the UI.  When users mouse over some of the table headers (shuffle read, write, and input size) as well as over the "scheduler delay" metric shown for each stage, a black tool tip (see image below) pops up describing the metric in more detail.  After the tooltip mechanism is added by this commit, I imagine others may want to add more tooltips for other things in the UI, but I think this is a good starting point.

![tooltip](https://cloud.githubusercontent.com/assets/1108612/3491905/994e179e-059f-11e4-92f2-c6c12d248d81.jpg)

This looks scary-big but much of it is adding the bootstrap tool tip JavaScript.

Also I have no idea what to put for the license in tooltip (I left it the same -- the Twitter apache header) or for JQuery (left it as nothing) -- @mateiz what's the right thing here?

cc @pwendell @andrewor14 @rxin



It might be good to mark this \@DeveloperApi (or make it `private[spark]`)