More improvements

Here's a new set of improvements by @mivanek and me, as promised in #7. 

Some of the added features include:

* accept multiple gems at once
* accept a gemfile which will be used
* removal of unnecessary stuff from Gemfile
* removal of unnecessary folder from version control
* new class for argument parsing
* new runner class
* messages are extracted into seperate module
* use a class for dealing with gems (some methods from runner should probably be moved there)
* cover everything with tests

What would be nice to have:
* `--no-version` option, adds into gemfile without fetching latest version
* `--version` option for specifying version
* `--no-install` option, skips `bundle`
* `--environment` option, accepts multiple environments
* check if the gem's already been included inside gemfile
* force new version of a gem
* global `~/.gemrat.yml` and local `.gemrat.yml`

@mivanek is currently working on adding the option parser to *Arguments*.

Thanks for the great gem!