Use Grunt's resolved file names if there are any

Currently the only way to define what tests to run are by defining a folder and then a mask with a glob-pattern that is appended to that folder.

But it is also possible to tell Grunt the file patterns, like:

```javascript
{
mocha_istanbul : {
      options: {
        ui: 'tdd'
      },
      basic: {
        src: ['test/**/*.js', '!test/integration/**/*.js']
      },
    }
}
```

And Grunt will then itself resolve what files to include and set the actual file names in `this.filesSrc`. Using the Grunt property rather than deferring the discovery of files to Mocha has the advantage of more easily being able to exclude certain files.

And as Mocha supports a list of files one can just as well send the Grunt list into it as sending a single folder into it.

Doing so also makes the module behave a bit more familiar to those used to other Grunt tasks.

This pull request add the ability to send the Grunt resolved file names into Mocha, while still preserving the old behavior of combining a folder with a mask. What's your opinion on this change? Looks okay?