Raster implementation for atmega328p

Hi Stefan, 
This is a blind port of the raster code from my Cortex-M4 fork. 
I've re-written it to take incremental block updates (up to 70 pixels per gcode line) so that the memory overhead is reduced. 
This is completely untested - I have just made it compile and link within the memory footprint limits (2K RAM is not very much!). 
I've added my eclipse project and linker script for reference.

There are also one or two cosmetic changes, and a couple of bug fixes that I needed:
 - read float, which was interpreting G0X10Y10 as hexadecimal G"0x10"Y10
 - a rounding bug when calculating adjusted_rate that caused the MCU to crash and laser stay on!

Cheers,
Richard.