Implement allocating IPs from CIDR within bridge network 

Fixes #4986

It is similar to openstack nova `--fixed-cidr` and allows to do things like [this](http://goldmann.pl/blog/2014/01/21/connecting-docker-containers-on-multiple-hosts/). And our network engineer very excited about this change :)
Also I've refactored ipallocator a little more.
I'm actually don't follow. `docker0` and "it" are in different sentences and `docker0` and bridge are synonyms anyway. Could you provide exact text for me?