Adding Rake as development dependency
Let the user specify the group for the gem being added.

Added the `-g` version that lets you specify which gem group the gem should be
added to.

```sh
add_gem unicorn -g production
```
Mark it as v 1.0.0

This is stable enough to be 1.0 IMO
Merge pull request #2 from nhocki/allow_groups

Let the user specify the group for the gem being added