Add Rakefile with bundler install tasks

This adds a `Rakefile` with the following tasks, which make building and releasing new gems a little easier:

```sh
> rake -T
rake build    # Build gh_contributors-0.8.0.gem into the pkg directory
rake install  # Build and install gh_contributors-0.8.0.gem into system gems
rake release  # Create tag v0.8.0 and build and push gh_contributors-0.8.0.gem to Rubygems
```

Feel free to close if you prefer to perform these tasks manually.