Added 9-bit frame support to Hardware Serial

I've added support for sending and receiving 9-bit frames for AVR based boards.
The changes are tested with a test sketch [1] and also with a logic analyzer (Saleae Logic 8).

I use two bytes in the buffer for storing a 9 bit frame. So i don't have to change the buffer type to uint16_t which means that all other modes then 9 bit don't waste RAM. The only downside for 9 bit users is that the buffer has only half the size.

[1] Test Sketch