JoyStick library

I've added a library to handle a joystick with 2 axis connected to analog pins like this one:

![Thumb Joystick Module for Arduino - Black5-500x500](https://f.cloud.github.com/assets/559456/143217/3c783cd0-7382-11e2-972b-b845ccb388dc.jpg)

It triggers events depending on the axis and returns a percentage of movement in the event direction and the real voltage value.

Added the description to the Readme.