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.
@mgcrea I took a look at Bootstrap's implementation.  From what I can tell, how Bootstrap auto positions and how I visualized the auto positioning is different.  

When auto placing in bootstrap: 
- It doesn't give you the option to specify a desired placement.  If I want my tooltips to appear on the top of the element unless it can't fit, Bootstrap doesn't let me.  
- It positions based on the container and not the viewport. 
- Bootstrap doesn't support the exotic placement

Thoughts?  Do we follow the Bootstrap implementation?  Role or own?  Give an option for positioning via the viewport vs the container?