Pushed a new firmware branch *fix-avr* to github. The JennyPrinter port made the avr/atmega side to slow.

Changes are:

* Integer math instead of floating point.
* Reworked usb-serial interface: store 512byte blocks.
* Removed compression with zlib, the avr has not enough cpu cycles.
* Experiment: auto-baudrate. Switch between 1000000, 500000 and 250000 baud.
* SDReader: double-buffering.
* Many other improvements and cleanup.

So for now, fix-avr is the branch to use for avr/atmega based printers and master is for
the stm32 JennyPrinter. Branch fix-avr has will be merged into master.