Add http agent and fix agent helper. (#42)

[suphoff]

Example http agent usage:

pom_battle --agents simple::null,simple::null,http::localhost:8085,simple::null --render

PR is mostly a copy & paste from docker agent and some fixes for broken agent selection CLI. Would be nice to use it as a base for the docker agent to avoid duplicated code - but don't really have the time.

[suphoff]

Mhhh - addressed cinjon's requests a while ago, rebased and squashed and forced push. Will investigate later today why it does not show up.

[suphoff]

OK - changes are in now. In future I will just add to the commits and count on commits being squashed on merge. Forced pushes are just evil.

[cinjon]

Thanks! Agree re force pushes.