Added multi address slave

I've removed the pullups enable in twi.c and I've moved it in Wire.cpp in order to make twi.h/twi.c usable in pure avr-libc projects
I've also added the multi address slave capability; the device can now respond multiple addresses (when configured as slave), and know which one was called thanks to twi_slaveAddress (in twi.c) or TwoWire::slaveAddress (in Wire.cpp). This feature is not supported on uCs without the TWAMR (two wire address mask register), like the ATMega8 (used, for example, in the Arduino NG), and it is disabled with an #ifdef