[STM32F3XX] Initial port

This is the initial port of the mbed SDK to STM32F3XX (Tested with STM32F3Discovery and hello world test.)

This port is based on the NUCLEO_F302R8 target.
Only the GCC_ARM toolchain has been tested.
Merge pull request #255 from bentwire/master

[STM32F3XX] Initial port