Sym link mods

This PR includes commits from #13 from @Aethylred because of puppetlabs/#160.

It also includes a few additions:
- Spec tests and modifications to meet specs
- Changing the name of the symlink titles from `enable.${mod}.load` to `${mod}.load symlink`
- Management of `mods-available` directory on Debian OS families
- Addition of `apache::mod::proxy_html` template (was absent) and removal of libxml2 hacky stuff for proxy_html because libxml2 is not required for the proxy html httpd module and it was complicating the mod design. An `apache::mod::libxml2` class should be created for the addition of this.