stm32f3discovery examples added.

- miniblink
- button
- fancyblink
- adc
- Non tested yet! But they compile.
stm32f3discovery ld script corrected as recommended in #82.
Flashing now using openocd. GDB Debugging possible and tested :).
Adc example fixed (printing garbage before) and cleaned.

- Integer printing function fixed.
  	  - because of comparison against smaller than 0 numbers we must use signed numbers!
	  - Value zero was not printed at all.
	  - It was printing one more character than it should.
- Erasing unnecesary commentaries.
- Erasing debugging help.
stm32f3discovery: Adding i2c accelerometer example.
stm32f3discovery: Moving read and write convinience functions to libopencm3.
stm32f3discovery: spi l3gd20 example added.
stm32f3: cdcacm example added and tested.
stm32f3: Examples Makefile now includes f3 dir.
stm32f3: cleaning some unused variables.
[STM32F3] Updated locm3 so that the newly added f3 examples compile.