Merge pull request #44 from hadleyrich/master

Fix up missing C++ wrappers and whitespace on newer drivers.
Missing semicolon in usart values when using 2MHz clock
Allow passing F_CPU from make params
This allows you to co-ordinate frequency between this lib and your project during build
Add support for listening sockets and accept/rx callbacks.
Still to do: disconnect callback.
Fix interupt handling.
Must do very early reset of interrupt status register to ensure further interrupts result in a falling edge.
Remove some debug which crept in.
Switch to single callback for all socket events.