Package command: using composer

Package command to manage community commands. It uses composer to manage the packages and provided wrappers around these composer commands to allow for easier installing/removing of commands.

By leaving composer intact in the `~/.wp-cli` directory it remains possible to do more complicated package management with composer itself when navigating to the  `~/.wp-cli` directory.

This way we provide a nice and simpler interface for package management but we also provide the maximum power of composer itself.

Current features:
- can install commands from packagist
- can install commands from VCS: git, svn, mercurial (any type composer provides is possible)
- can also install commands from private github/bitbucket repositories. No limitations, again composer supports this so we support it.
- manage complicated packages directly in  `~/.wp-cli/composer.json` or by using `$ composer` in `~/.wp-cli/`

Todo:
- Add Satis support to easily find community commands? `search` subcommand?
- Composer's output is current not displayed, maybe suppressed by WP-CLI? @danielbachhuber 
- How to test the phar version of wp-cli? I'm not sure about the paths in that instance @danielbachhuber
- I didn't really know when to load the commands, I've added it to the runner. Maybe that should be done in a nicer way? @danielbachhuber 

I hope you like the approach. If not, I'm looking forward to the feedback :)