Merge pull request #18 from hadleyrich/master

Refactor USART hw descriptiors
Closes #17
Merge pull request #19 from hadleyrich/master

Update usart examples to match new API
Define familes for all known XMEGA chips.
Define serial port maps for all known XMEGA chips.
Port SPI interface to use hardware names, all known XMEGA chips covered.
Fix assigned family for 256A3B.
Actually implement SPIE and SPIF support.
Oh hey, let's actually support all the USARTs really ><
Add C++ guards