Multiple reporters support (continuation of #930)

This is a continuation of @misterdjules PR #930 – multiple reporters support.

On top of @misterdjules work, it has:
* Restores 'spec' as the default reporter
* Fixed unit tests
* An additional unit test for multiple reporters (tests/reporters/multiple.js)
* Renames variables to be closer to original mocha.js code (e.g. `_reporter` -> `_reporters`)
* Keeps original `Mocha.prototype.reporter` interface (vs the plural `reporters`) for API compatibility. IMHO anybody using Mocha directly shouldn't have to change their code to remain compatible.

It does not implement Growl as a separate reporter, as was discussed [here](https://github.com/visionmedia/mocha/pull/930#issuecomment-48971366). I think it could be done as a follow-up patch just fine, but if you'd prefer that was done before accepting then that's cool too.