sched: core: implemented a prio change: nice()

`nice` is the traditional name of the function to change the priority of
the currently running thread or task. The nice in ETA/OS works as
follows:

	* save the old priority
	* set the new one
	* call yield to see if other threads became more important
	* return the old priority

Signed-off-by: Michel Megens <dev@michelmegens.net>
doc: abstracted all module documentation.

Added all the documentation for modules in a single file:
Documentation/etaos_en.txt

Source files only contain member documentation.

Signed-off-by: Michel Megens <dev@michelmegens.net>