Provides proper clean for the controller and robot

(not really tested yet)
with Robot(...) as robot:

Add the possibility to use the Robot through a ContextManager.
Updates the way closing Controller and Robot works (no __enter__ and __exit__ anymore)

Instead of defining __enter__ and __exit__, it’s probably better to
only define a proper close method and document how to use the closing
method from the contextlib module to do the work for us.
Document how to properly close and clean the Robot.
Merge pull request #18 from poppy-project/cleanup

Add the possibility to close and clean the Robot.