Gracefully fall back to plain ØMQ if CurveZMQ is not available

It appears to be impossible for us to distribute Bitmask via the Ubuntu Software Center if we require support for CurveZMQ. This is because the python zmq libraries in past, present, and future versions of Ubuntu do not support curvezmq and we are forbidden from depending on newer or alternate versions of any library that is already packaged for ubuntu.

Therefor, the easiest plan of action seems to be to fall back gracefully if we can detect that CurveZMQ is not available. This will would allow another user on the system to potentially steal the user's password.

The better solution would just be to rely on file-based TCP sockets with strict user permissions. Then we can use the simple and standard python TCP RCP library.

(from redmine: created on 2015-01-14, closed on 2015-02-25)