* resurrect lost purge broken files scheduler tasks

* some cleanup in comments and user facing text
* inline simple private method used only once

in https://github.com/sonatype/nexus-ruby-support/issues/55#issuecomment-60445832 I had the feeling it is not clear which scheduled task is meant for the hosted repo or for the proxied repo. this is meant to help to make the intension clearer - I hope.

somehow - properly I wanted to delete it - the purge-broken-files tasks is not there anymore. this patch will resurrect it, since the under lying code still exists and gets tested.
Good point... this shows an dysfunction in the api where there are no impls of a local storage that aren't filesystem based, yet we force this crazy abstraction upon places where they actually need files and paths.

Either way this code that uses replace to strip off the scheme from a url to determine to file path of the storage directory is pretty horrible and retarded.