added: spec file for redhat packaging

This adds redhat (centos) 6.x packaging for opm-core.

I had to package some predeps, the specs and patches required for those are (temporarily) available at git://github.com/akva2/APEL.git

APEL refers to roland's toungue-in-cheek 'arnes packages for enterprise linux'

the pre-steps i did in a hurry, so most of them will likely require revisiting. i don't know of any issues with the opm-core file in the pull request.

i stuck it in a directory on the top of the tree called 'redhat'. there is no strict requirement of this as there is no such thing as in-tree packaging of RPMs. as such, the file can be moved around wherever you want, including a separate repository should you prefer that.