Add TWI Peripheral Driver for XMEGA

I added a peripheral driver for the XMEGA platform.  I've been using it with a XPlained A3BU for the last couple weeks and debugging / testing it with a MPU 9150, it seems to work.

I've tried to keep the same API as the AVR8, with the obvious addition of the TWI_t pointer to denote which twi bus to work on.