Cosa/USI based TWI slave driver for ATtiny

Cosa TWI (I2C) slave device driver for Arduino and ATtiny. Allows easy design of TWI slave devices using a simple event callback design. The Cosa TWI ATtiny driver uses USI and implements variable write blocks with STOP detection. See CosaTWImaster/slave for an example of usage pattern; and the example sketches CosaTWImaster/slave and CosaTWIremotePinMaster/Slave.