Added the idea of single run to grunt for Karma

Thought that tying single-run to isDev was a bit opinionated after a conversation with @ironsidevsquincy.
It also got stuck if you were committing in dev mode.

You can now use the `--single-run` option, default set to `true`.

One other thing to note is that you cannot run in `no-single-run` mode if you want to run `test:unit`. You need to specify a subtask for the same reason of getting stuck. i.e. you have to run `test:unit:discussion` or an equivalent.

![Karma will come back](http://cdn.gifstache.com/2012/10/12/gifstache.com_2286_1350172800.gif)