USCI I2C support.

Current limitations and features:

* Master mode only.
* No multi-master bus configurations.
* Will automatically reset a hung bus, if for example the master reset in the
  middle of a transaction and a slave currently is driving SDA low.
* Repeated starts are supported and can be very time efficient.
* An I2C transactions may be processed with multiple I2CPacket.read() or
  I2CPacket.write() commands, as appropriate.  The first must include I2C_START
  and the last must include I2C_STOP.
* Conditions exist during read where the hardware may clock out an extra byte
  from the slave before stop.  This byte is not passed to the user.