Add support for tracking ownership of tags to the tagpool manager.

It's often useful to know who a particular tag was acquired by (so that one can figure out who to prod about releasing it) and what tags a particular person has acquired.

Outline of proposal from IRC:

> On the ownership front, I was thinking of adding an owner parameter to the acquire tag 
> function and a command for listing who owns a tag and the tags a person owns. Leaving 
> the owner as None would make the tag own by a special null owner.