Extract format module

Extract formatting utils used by unhandled rejection reporting, and add some unit tests.

See also #412 
No worries. Thanks for bringing this up, actually.  This is the standard pattern we use for our unit test modules, and it's something of a throwback.  BusterJS adds itself as a window global in browser environments.  However, we browserify all the test modules before running the unit tests in browsers (all automated on Sauce Labs).  So, as you said, we may not need the browser global check anymore.

We'll have to test that.  It'd be nice to get rid of the global check and just go with CJS.  Thanks!