Tape

Currently, Underscore is not routinely tested in a node environment or across multiple browsers unless it's done manually.  This causes errors to [slip through](http://git.io/1C2YYQ) undetected (I point this out to illustrate that they slip through as we all make errors of this kind from time to time).  In an attempt to improve the testing situation a bit, this patch replaces QUnit with [tape](https://github.com/substack/tape) with the following benefits.

* Tests can be run in a commonjs environment like node.
* Automated cross browser testing via [testling](http://testling.com) ([example from my fork](http://ci.testling.com/braddunbar/underscore)).

*Note: Running tests now requires a build step, something we can probably work around if necessary.*