Added try/except for setting signal handlers.
Added try/except for setting signal handlers.

Setting signal handlers from inside a thread is not supported in Python,
but some applications need to run Sleek from a child thread.

SleekXMPP applications that run inside a child thread will NOT be able
to detect SIGHUP or SIGTERM events. Those must be caught and managed by
the main program.

EnerNOC/smallfoot-sleekxmpp,skinkie/SleekXMPP--XEP-0080-,destroy/SleekXMPP-gevent