simplify istanbul invocation via grunt-mocha-istanbul

Of the grunt+istanbul plugins, I found [grunt-mocha-istanbul](https://github.com/pocesar/grunt-mocha-istanbul) to be the easiest to use. It's a lot more "grunt-y" than your solution and adds easy config for other options.

The only part I'm not completely crazy about is the way you have to specify `src` & `mask`. Seems like that could be done better, to provide a glob-style value for `src`.