Add Initial Support for STM32F4DISCOVERY Board

* Add a TARGET definition to Makefile (default PYBOARD).
* Add support for discovery LEDs in led module.
* Add support for discovery user switch in usersw
* Add EXTI interrupt handler for discovery user switch on (PA0).
* Parameterize led and usrsw modules pins and port.
* Issue #83
Add gitignore file
Merge pull request #86 from iabdalkader/master

Add Initial Support for STM32F4DISCOVERY Board