MCO-421 Do signal processing in threads

In ruby 2.0 and onwards it's not permitted to do anything that would
block in a signal handler.  As our control flow doesn't have an
obvious place to process a raised flag, we instead take the approach
of spinning off a new thread to do the processing.

Tested on ruby 2.1.2
Merge pull request #225 from richardc/MCO-421_ruby2_signal_handlers

MCO-421 Do signal processing in threads