allow repository add-ons to point to multiple add-on repos, versioned by xbmc.addon version.
[cleanup] CRepository FetchChecksum can be static, some other methods const, and there's no need for a critical section.
[cleanup] includes in Repository.h/cpp
[cleanup] drop CStdString from CRepository
add the gotham repository to repository.xbmc.org, enabled for versions greater than 12.9.0