Support for RHEL / CentOS using the EPEL repository

This adds support for RHEL and CentOS when using the EPEL repository. For the most part, gearman is setup very similar to that of Debian family.

I couldn't get the specs to work properly when looping an array for `$::osfamily` names so I just took the easy way out and made separate contexts (this is my first jump into RSpec).

I'm not sure if it makes more sense to include the epel module directly, or notify the user instead with something like:

```ruby
  if $::osfamily == 'RedHat' {
    if ! defined(Class['epel']) {
      fail('You must include the epel module!')
    }
    $require = Class['epel']
  } else {
    $require = undef
  }
```

On my local workstation I use RVM and was thinking about including the `.ruby-version` and `.ruby-gemset` files I created to more easily test with Ruby 1.9.3 and a clean gemset (I had some conflicts with existing gems in my main gemset).

I'm also testing with Vagrant and have a Vagrantfile to bring up a CentOS 6 and Ubuntu 12.04 and was thinking of including it.

Let me know what you think, thanks for the gearman module :smile: