Add drush command proxies

This means (almost) any drush command can be run from inside the REPL :)

![cache-clear](https://f.cloud.github.com/assets/53660/2063398/c21d0a3a-8cb5-11e3-9bbe-ad340d5e296f.png)

![help](https://f.cloud.github.com/assets/53660/2063365/39832934-8cb5-11e3-8ee8-368631d972fe.png)

![more-help](https://f.cloud.github.com/assets/53660/2063378/6ca70042-8cb5-11e3-9b57-7ddd7febcefe.png)