Added source code for a PID controller from Atmel and changed the Makefile to accept more than one input file.
Added routines for using the USART.

USART is needed in order to communicate to an FTDI FT232 chip that connects to a PC through USB.
Included the new USART code into the program and transmitting the measured ADC value after every conversion.
Changed the ADC to use AVCC as voltage reference. This makes it necessary to perform the differential measurement with gain 10x instead of gain 1x.