Add EPEL support.

While testing centos5/6 I realized it was going to fail as we require
EPEL to be enabled and the testing images don't have it.  Add
`epel_enable` as an option in case people are already handling epel
in another way.

This fixes the centos6 case which didn't work out of the box.
Flip the epel_manage option to false by default.

I talked to #puppet about this, conclusion was most people are
going to be managing EPEL anyway so they won't need this.
Fix specs after flipping defaults.
Merge pull request #3 from apenney/specs

Add EPEL support.