Add rubocop to check code quality

[RuboCop](http://github.com/bbatsov/rubocop) is a Ruby static code analyzer. Out of the box it will enforce many of the guidelines outlined in the community [Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide). 

Custom stored in the `.rubocop.yml`
* Mute documentation check
* Force SymbolArray check