Changes for Step5

USB library added (not yet completed)
Build settings can be overwritten in custom Config.mk file now (by abidon)
.gitignore added (by abidon)
Improvements to Circle and Angle:

- Generic doubly linked list implementation in angle
- Usage of the linked list to:
	- allow more than 20 timers.
	- sort timers by their duration until they fire
Syncing with rsta2/circle's Step5.

Merge remote-tracking branch 'upstream/master'

Conflicts:
	README.md
	Rules.mk
	sample/kernel.cpp
	sample/kernel.h