drivers: added LPS331AP pressure sensor driver
board/iot-lab_M3: added interface for LPS331AP
tests: added test for LPS331AP sensor driver
core: refactor sched_run

`sched_run()` was cluttered. Many individual changes were done without a
proper refactoring.
Merge pull request #1817 from haukepetersen/add_driver_lps331ap

drivers: added driver for LPS331AP pressure sensor
Merge pull request #1841 from Kijewski/sched_yield-result

 core: refactor sched_run