add git-svn and wiki tooltip support

Hi davglass,
I'm working on a fork of your github-trac plugin for the Parrot VM that I'd like to get merged.  We (the Parrot VM, parrot.org) are migrating our vcs from svn to git and the goal of my modifications is twofold:  First, I want to make sure that (as much as possible), no links to the svn repo in trac break.  My changes accomplish this by storing a map between svn revision numbers and git ids in a db table, using those to map between trac changeset urls and github commit urls.  Second, I don't want to lose tooltips for links to commits.  The implement this, the fork also stores commit messages in the db table, exposing them when it detects a valid svn revision or git commit id.

I have a demo site set up at http://mksig.org/trac, though my vm is underpowered for trac and makes the site slower than I'd prefer.  The plugin does slow down wiki rendering, but it's configurable and doesn't carry too stiff a penalty.

I'd appreciate any feedback you have as I'm fairly new to Python programming.  If you feel like the code is good enough to merge as-is, that's great too.

Thanks,
Christoph