Implemented Rubocop

Pulled in [Rubocop](https://github.com/bbatsov/rubocop) with some [cops](https://github.com/bbatsov/rubocop/tree/master/lib/rubocop/cop/style) disabled. Decided to take a stab at running the StringLiterals cop as an example of the excellent job its auto-correction does :+1: 

To continue with this, we'll need to fine tune which cops to run with and which to disable.

Personally I'd like to see Rubocop be pulled in, ran after each successful suite but not failing the build (for now). As there will be lots of false negatives until we get '0 failures' whilst running Rubocop.

Thoughts?