Add Codebreaker binary

Make executable: chmod +x bin/codebreaker
Run: bin/codebreaker
Define and satisfy 'Given the secret code is' step

* Pass in secret as argument to Game#start.
* Revise unit specs to call #start with secret.
* Revise earlier 'When I start a new game' for #start to take argument.
Define and satisfy steps for simplest guess scenario

* Defined the "When I guess' and 'The mark should be' scenarios
* Built unit specs around simplest case (no matches).
* Passed unit specs and simplest guess scenario.
* Refactored unit specs and production code.