#62 solve celery incompatibility

[sebastiendarocha]

Replaces multiprocess by billiard

Tested on python 2.7 and 3.5

[mpeychev]

@pnpnpn Any thoughts/comments?
For my usecase this fix results in
OSError: [Errno 24] Too many open files
both on Mac OS (with ulimit -n being 256) and Ubuntu (with ulimit -n being 1024).

[mpeychev]

I opened another pull request ( #67 ) for the above issue that builds on top of this PR.