CentOS/RHEL/SL support

Added `rpm` packages support for RHEL, CentOS and Scientific Linux. Fedora has **not** been tested.

To test, same drill:

    pip uninstall -y fabtools ; pip install git+git://github.com/scalp42/fabtools.git@rpm_support

I would like to start refactoring some code once this is merged (to reduce extra merging work), especially the `distrib_*` and `require.package*` methods in `deb.py` and `rpm.py`.

Also, please note that `sudoers` is *not* installed by default on RHEL and CentOS. Not sure if we should handle it, as it should return output and be handled in `fabfile.py` I think:

    sudo command: not found

Please have a look, feedback welcomed.