Add set_mx_type and del_mx to Lumberg::Cpanel::Email

Adds ability to delete mx records via [delmx](http://docs.cpanel.net/twiki/bin/view/ApiDocs/Api2/ApiEmail#Email::delmx) and set the MX server type (auto, secondary, local, remote) via [setmxcheck](http://docs.cpanel.net/twiki/bin/view/ApiDocs/Api2/ApiEmail#Email::setmxcheck).