add session cleanup utility
Merge branch 'devel' of github.com:radical-cybertools/radical.pilot into devel
fic #244
fix #113
make sys.exit usable in callbacks

This uses a somewhat crude way -- but since all threads are daemons anyways, that won"t make our exit handling any worse.  This should be revised once we improve the thread managemenent.
better exit code in example
Merge branch 'devel' into experimental
scattered fixes toward cleanup and working examples