Merge work done in grunt-simple-mocha to this plugin

I'm opening this pull request to propose merging work done in [grunt-simple-mocha](https://github.com/yaymukund/grunt-simple-mocha) into this plugin. The patches I've created incorporate the functionality to this plugin in the following manner:

1. If the list of files passed contains no .html files, the tests will be run in node.js
2. If the list does contain .html files, PhantomJS will be used instead
3. If the multi-task target is configured with _type: unit_, the tests will be run in node.js

This pull request was inspired by having to deal with the two plugins sharing task names (see some of the forks of the grunt-simple-mocha plugin). I'm also attempting to contact the author of grunt-simple-mocha to have him collaborate on this plugin instead on the node.js testing part.