Some updates to signal handling.

* SIGTERM and SIGQUIT are switched to better conform with unix
  conventions.  TERM is now used to gracefully wind down and QUIT is now
  used for immediate shutdown.

* Instead of using SIGINT for halting processing we now use SIGTSTP.
  Handling SIGINT meant that any sort of KeyboardInterrupt handling
  broke as the INT signal begets the exception.  The TSTP signal is a
  handy "interactive stop" signal that fits the bill for the intended
  behavior nicely.

  As a consequence of this doing "^C" in a terminal where a rotterdam
  server is running will now gracefully shut down the server.

* Add a "pause" facility to the control script.  merely sends the TSTP
  signal to toggle processing of jobs.
Eliminate the use of __all__.

Turns out that not only was I using it wrong (it's only supposed to be
an iterable of strings apparently) they also didn't behave the way I
thought and were only applicable when importing via "*" which should
never be done anyway.
Change the config file option from -c to -f.
Fix typo in server setting CLI name.
Add "in" operator functionality to Config objects.
Introduce optional concurrency feature.

If gevent is installed, a server will be able to have a set number of
green threads per consumer process, multiplying the total capacity by
however many green threads are set.

Very useful if the jobs in the handled queue(s) tend to be I/O heavy.