Describe features and scenarios for first iteration

Features:
  code-breaker starts game, and is greeted.
  code-breaker submits guess, and receives feedback on guess.

Features have not been defined. Step definitions to be provided.
Add pending step definitions for features
Satisfy given and when steps of codebreaker_starts_game
Complete codebreaker_starts_game.feature

* Define output steps, using injected `output` dependency.
* Add unit specs for Game#start output.
  * Add spec_helper
* Fulfill expectations by rendering output from Game#start.
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.
Remove code repetition in spec
Satisfy '1 exact match' scenario
Satisfy '1 exact match' scenario
Satisfy all '1 match' and '2 matches' scenarios

* 1 number match
* 2 exact matches
* 1 exact and 1 number match
* 2 number matches
Correct scenarios

The output for some guesses in the 'all numbers correct' section
were specified wrong.
Satisfy all guess scenarios

Exhaustive unit specs were not necessary to cover all cases.
Extract method object

The Game class was overloaded with two responsibilities:
1) Handle game play.
2) Evaluate guess.

The second responsibility was moved to a new class, Marker.  Specs
for its behavior were migrated to the new class, and a spec was
added to show that Game finds the mark and outputs it.
Use #let to indicate marker setup

Set apart logic instead of keeping it in the before block.
Setup Game's marker at start of game

Since the secret only changes at the beginning of the game, it isn't
necessary to make a new marker every time the user submits a guess.
This change allows us to do away with the @secret instance variable.
Refactor to remove repeated code

and express intent.
Adjust game-play script to take guess input
Merge pull request #1 from amar47shah/simplest

Implementation of First Iteration