* 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.
the current ruby implementation of rebuilding the metadata files acts on the file system and needs the root directory. it is actually functionality which comes with rubygems itself, i.e. there is not much code on nexus side.

first I thought it a huge tasks to move it to java side of things. but I think it is easy to move all those schedule tasks to the java side and use the usual repository/storage API. but this is a bigger change altogether.