Changes on keyboard press are incremental.

The previous implementation naively set the value of the force of the
appropriate thrusters.

Now, a subscriber is declared inside the same node. And this subscribes
to the present value, and when a new change has to be done, the value of
the force is either increased or decreased from the earlier value.
Added the top and bottom keybindings.

Also, moved everything to a dictionary, to avoid endless if-else
ladders.
Add STOP button. Bound with Space key.

Pressing the space key will kill all the thrusters.

The vehicle will come to a rest after it loses its inertia.
Add the actions for STOP button.
Added the keyboard shortcuts in the button text.
Merge pull request #37 from icyflame/keyboard-control

Keyboard Control: Improvement