Support storing job config history in folders

This plugin seems to be completely broken with respect to the CloudBees Folders plugin (or generally with `ItemGroup`s other than `Jenkins` itself). I suspect this is a regression caused by the recent refactorings.

Functional testing using `MockFolder` only possible when using a core dependency ≥ 1.494.

The patch here should be considered a hotfix: it just hardcodes the fact that `jobs` is the directory separator for folders and `job` is the URL separator. (These names match those defined in Jenkins core.) For the directory separator issue, probably `AbstractItem.getConfigFile` or `PersistenceRoot.getRootDir` should be used rather than the current hack of `JOBS_HISTORY_DIR`. For the URL separator issue, `AbstractItem.getUrl` is probably what you want.