Added sequencer timer&interrupt with variable BPM

Added delay functions based on SysTick
Added ugly debugging code in main.c/.h
Added structs for instruments and sequencer.

Cleaned up a few minor things.
GPIO no longer simply toggles but is switched off after 5ms (independent of system clock) to conform to analogue trigger specs.
Sequencer now actually sequences!

GPIO D 12, 13, 14, 15 now output trigger signals for 5ms.
digidrum hi-hat
Merge remote-tracking branch 'origin/Digi_drum' into Sequencer

Conflicts:
	drHUmmer.coproj
	interrupt.c
	interrupt.h
	main.c
	main.h
	timer.c
Properly merged sequencer and DAC output
Included digital sounds

	triggered by sequencer
	output by DAC

Adjusted interrupt routines
BPM now controlled via ADC
Added two more digital samples (basskick & cymbal) and incorporated them in the program.

Also made the sample arrays into constants to spare the RAM (they're written to Flash instead).
Implemented UI functions to sequencer.
Corrected steps vs BPM

Added substeps (6 per step)
Adjusted structs accordingly
Separated into normal operation & debug mode (#define DEBUG in main.h)

Added circular buffers and functions to write & read them
Debugging adjustments. No functional changes.
Merge branch 'Sequencer'