Add ember install commands

Adds 3 new commands

1. `ember install`
2. `ember install:bower`
3. `ember install:npm`

They behave exactly as you'd expect. Install runs npm and bower install on the
project. The last two simply pass in the package names you give it to the
underlying task to do it.

closes #2695
I wasn't sure. I suppose if it ends up prompting the user that could cause some weird behavior