Added the idea of single run to grunt for Karma

Thought that tying `single-run` to `isDev` was a bit opinionated and also got Grunt stuck in a few bits. This is after a chat with @ironsidevsquincy.

You can now use the `--single-run` option, default set to `true`.

If you'd like to run Karma in `no-single-run`, run `test:unit:subtask --single-run=false`.

One other thing to note is that you cannot run in `no-single-run` mode if you want to run the whole `test:unit` task.
You need to specify a subtask for the same reason above of getting stuck.

i.e. you have to run `test:unit:subtask` or an equivalent.

If all is good, I'll email the change around.
![Karma will come back](http://cdn.gifstache.com/2012/10/12/gifstache.com_2286_1350172800.gif)