Added damage-based regen, and support for AFK.

* Added soap_dmg_heal_ratio. If you set it to 0.5, and you do 100 damage to someone that dies within the next 10 seconds, you will get a 50 hp boost. (It has been running for a while on my server, and people are beginning to [request it](http://etf2l.org/forum/customise/topic-27485/page-8/#post-509098).)
* Added support for a not yet released AFK plugin. Whenever someone is AFK, they will be teleported to the spawn room. When they return, they are teleported back into the battlefield. (Only if the AFK plugin is installed.)