add support for mail module

See http://wiki.nginx.org/Modules#Mail_modules

Sample Usage:

class { 'nginx':
  mail => true,
} 

 nginx::resource::mailhost { 'domain1.example':
   ensure      => present,
   auth_http   => 'server2.example/cgi-bin/auth',
   protocol    => 'smtp',
   listen_port => 587,
   ssl_port    => 465,
   starttls    => 'only',
   xclient     => 'off',
   ssl         => 'true',
   ssl_cert    => '/tmp/server.crt',
   ssl_key     => '/tmp/server.pem',
 }