Fix #216

This fix lock-ups on UART errors (for example when disconnecting
and reconnecting RX/TX wires or if the method Serial.begin is
called while another device is already transmitting).

Signed-off-by: Thibaut VIARD <thibaut.viard@atmel.com>

 WIRE class finished. SERCOM class completed. Few correction in HardwareSerial and SERCOMUsart after compilation

Add SPI functions into SERCOM class and create SPI class