Use forked pygit2
Use prebuilt pygit2 instead
Use RotatingFileHandler. Closes #144
Remove duplicated sip api settings
Add rotater format and remove some padding
Mod.path is now relative
No more serialized paths

I renamed mod.path to mod.identifier.

Now, it ought to be some kind of unique identifier -- we could go with what's
in mods' mod_info.lua, but it's neat to have kind canonical short names for
often used mods (faf, nomads), so I'm not sure what to go with.
Fix broken/failing tests and implementation
Wanted: Static verification of comment validity
Fix typo in settings reference
Use the right config key
Always use dev config when not frozen
Add embedded pygit when running tests and building