Add trigger support

trigger presses detected by phasor, sent to master. Master processes
trigger press and sends IR to transmit back to phasor.
Ensure xbee sleep is disabled
fiddle with xbee flow control and sleep mode
Fix off-by-one error in TWI Master Read handling

The TWI master read handler would receive one more byte than intended.
This causes the master to issue a NAK and a STOP immediately after
receiving the last byte so no more bytes are received.
Add retry to failed TWI master transactions

Certain TWI master error conditions will be retried automatically:
- Bus arbitration lost
- NAK from slave

The entire remaining part of the transaction will be restarted up to
TWI_MASTER_MAX_RETRIES times.