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.

sleekxmpp/xmlstream/xmlstream.py

@@ -199,11 +199,15 @@ class XMLStream(object):
         self.auto_reconnect = True
         self.is_client = False
 
+        try:
             if hasattr(signal, 'SIGHUP'):
                 signal.signal(signal.SIGHUP, self._handle_kill)
             if hasattr(signal, 'SIGTERM'):
                 # Used in Windows
                 signal.signal(signal.SIGTERM, self._handle_kill)
+        except:
+            logging.debug("Can not set interrupt signal handlers. " + \
+                          "SleekXMPP is not running from a main thread.")
 
     def _handle_kill(self, signum, frame):
         """