Feature/#20 multiprocessing support

[nesnoj]

Fixes #20

I added async multiprocessing, some questions to you, @timroepcke :

• The no of threads is defined in the config as intended by you. I'd propose to have it as arg in the CLI instead, what do you think? (this is not integrated yet)
• You had a DF charging_all for the result aggregation. Due to the asynchronous operation I had to comment this out. Is this still needed? If so, we have to implement this differently.

Please review #19 first as this PR already includes all features of #19.

[nesnoj]

I've tested this for a while now and it works well. May I merge, @timroepcke @Mosc5 ?