get rid of useuploadlimit and downloadlimit

I think at the commandline there is the parameter usedownloadlimit and useuploadlimit very unsexy. And in python we have the possibility to use None to show not to use up-/download limits.

But I can't test this, 'cause I don't have a configfile without a BWLimit section.
When you think this is working, than also README and manpage has to be updated.