Cleanup; add licence; turn into installable package

This pull request adds a GPLv3 licence notice (as recommended [here](http://www.infoworld.com/d/open-source-software/github-needs-take-open-source-seriously-208046?page=0,2) and [here](http://tieguy.org/blog/2012/12/03/licensing-confusion-is-great-for-lawyers/)), puts all functions in a namespace, and makes it possible to install riak-mode with `M-x package-install-file` - no load paths or requires required.