These are a pair of library files for the Arduino IDE that will allow the Leonardo, and any other board using the Atmega32u4 chip, to work as a HID joystick.
To install the libraries go to the arduino install directory, and replace the original files in hardware\arduino\cores\arduino\ with the ones here. Or just rename the original files and add .old to the end to keep them.
This is all based off of Connor's work located at http://www.imaginaryindustries.com/blog/?p=80 . All I really did was update the library files to be compatable with version 1.6.3 of the arduino IDE. Mostly just copy and pasted things to be honest.
Everything is released under the same license as the original arduino files.