Added support for Sublime Text 2 packages

Hey, 

I added support for Sublime Text 2 packages, so that you can do something like this:

```
include sublime_text_2
sublime_text_2::package { 'Emmet':
  source => 'sergeche/emmet-sublime'
}
```

There is one problem though. In case one has already installed the package using Package Control, one will get an error like below:

```
Error: Execution of '/opt/boxen/homebrew/bin/git clone --recurse-submodules -c credential.helper=/opt/boxen/bin/boxen-git-credential https://github.com/sergeche/emmet-sublime /Users/janne/Library/Application\ Support/Sublime\ Text\ 2/Packages/Emmet' returned 128: fatal: destination path '/Users/janne/Library/Application Support/Sublime Text 2/Packages/Emmet' already exists and is not an empty directory.
```

Any suggestions how to handle this?