feat(tooltip): add ability to auto position tooltip

Add ability for tooltip (and any dependent directives) to be auto
positioned.  Placement option has been enhanced to look for `auto`.  If
present, tooltip will try to position itself in viewport.  A preferred
placement can also be specified by appending desired placement to auto.
Tested against Win 7 Chrome, Firefox, IE 9-11.
I'll remove the loop.  I initially added the loop so it could keep trying to position itself in viewport but ended up restricting it down to 2.  Looking back at it, it doesn't serve any purpose.  If it's not going to fit in the viewport after the 1st attempt, it's probably not going to fit.

I'll explore the Bootstrap implementation and see if I can put the code into a private method.  Thanks for the feedback.