Fix suspend/start behavior

Fixes #77

OCCI.stop() is mapped to OS.suspend() and
OCCI.suspend() is mapped to OS.pause()

Both OCCI operations are inverted by OCCI.start(), which consequently
has to distinguish the current machine state to trigger the correct
inverse OS operation, i.e. OS.resume() or OS.unpause.
Merge pull request #79 from EGI-FCTF/fix_suspend_resume

Fix suspend/start behavior