Configurable storage backend

[holtzermann17]

This is also discussed in this ticket, but I am giving it its own home.

I'm experimenting with a dedicated "mob" branch (http://repo.or.cz/h/mob.html) and it would be nice to have ticgit integrated with that. (I know, I could host my own system with fully-anonymous push, but this seems like a cleaner and more useful fix!)

Edit: The idea behind this is to be able to configure how TicGit-ng stores it's information (tickets, state, attachments, etc), it should be able to store it in the branch as it currently does, as well as in the filesystem allowing per-branch instances, and perhaps even via a database.