Support simple Hg and Git storages

This PR is my first backup development attempt, so it may not conform to all contribution needs.

The backup is done on `package.trigger` rather than `package.filenames`, with (git|hg) commit.

The main problem to support SCM storages is that for them the packager, compressor and splitter doesn't make any sense at all. I had made efforts to make changes in the existing code as minimal as possible. See diff.
Is syncer called/useful for anything other than the spec suite? Does the configuration need it to be public?