Add support (and documentation) for yum priorities

The `gluster::repo::yum` class now accepts a `priority` parameter that
specifies a weight to apply to the repo.  This ensures that this repo is
evaluated before any others.

Closes issue #4.

Also adds a CHANGELOG, and bumps the version number in the metadata.json
file for the Puppet Forge.
Seems like your intent was not to specifically mention RedHat.  Might want to remove the reference.