retry polling if system got interrupted

might happen if run through gdb or system hibernation

I'm not 100% sure if this fix is really 100 good, as I don't know which real interruptions could happen where ws4py wants to stop.
But if you have cython compiled code with ws4py you can't use it with gdb anymore.
see as a few references:
http://stackoverflow.com/questions/2252981/gdb-error-unable-to-execute-epoll-wait-4-interrupted-system-call
https://github.com/mopidy/mopidy/issues/718