Add util for opening centered window (bug 1101002)

This adds:

* A util for merging defaults similar to _.defaults (albeit only takes one defaults obj.  
* The basic centered window opening code
* Plus tests for all of the above.
I was seeing them in my editor. Adding to predef fixed it. The jshint stuff that grunt is running won't be testing these files due to a typo here: https://github.com/mozilla/fxpay/blob/master/Gruntfile.js#L28 should be 'tests'.