Add a managed repo definition for the ELRepo.org repos

This adds a managed repo definition for the [ELRepo.org] repos.  It includes the GPG key.  The elrepo-release.noarch package I based it on didn't include source repos so I left them out, but they shouldn't be too hard to add later if somebody wants them.  I've given it a priority of 17, just below EPEL.

[ELRepo.org]:http://elrepo.org