Adapt admin interf. to change in agent model

Agent.state is no longer a simple int, so some of the special logic
attached to it has to change and some more had to be added.

In particular, it now has its own filter class.
Add use_address col. to agent view in admin interf.
Merge pull request #35 from guidow/agent_reduce_api_magic_nums

Adapt admin interface to change in agent model (no longer uses magic numbers)