Simplify RobotController, remove Command::Token dependancy

RobotController now responds to messages with the following
format: robot_controller.<token>(<args>)

ToyRobot::Application updated to use this pattern when
messaging the controller.

Resolves #15 - Remove Command:Token dependancy from
Controller
Refactor ToyRobot::View
Remove private method debug() from ToyRobot::Application

This method was untested and undocumented. It is of doubtful
value and thus has been removed from the code base.
Rename */test_command_matcher* to */test_command_parser*
Update README.md with Design section.

README.md now explains how the application is desgined/flows.
It also exapnds the Discussion section.
Delete ToyRobot::Placement class and tests

It was an unnecessary abstraction. All of the logic was refactored
into the ToyRobot::Robot class, which is a more natural place to
have it.
Rename RobotController to Controller

Its better since its more concise. It is also more aligned
with the naming of the View class.
Update README.md to reflect recent refactoring