Adds reporter and reporterOutput options

I wanted to use Grunt to automatically generate [TAP](http://en.wikipedia.org/wiki/Test_Anything_Protocol) files for Jenkins CI.  The current grunt-contrib-nodeunit tasks only runs the built-in grunt reporter, so it's not saving any of the test output.

I've added two options:
1) `reporter` specifies which reporter you want to use (defaults to the existing `grunt` reporter).
2) `reporterOutput` specifies the output file to save the reporter's output (optional)

Note this requires an update to nodeunit, so that all reporters accept a callback as their third parameter for their `run()` function.  Not all reporters prior to my pull request did this, including the `tap` reporter.  Here's the pull request:
https://github.com/caolan/nodeunit/pull/230

I version bumped and updated docs as well.