Add package method for SmartOS pkg_add

This package method interacts with SmartOS pkg_add to install from local or remote repositories. It is slightly different than the FreeBSD `pkg_add`.

 This example installs `perl5` from a remote repository.
 
     packages:
      "perl5"
      package_policy   =>  "add",
      package_method   =>  smartos_pkg_add("http://pkg.example.com/packages/");

 This example installs `perl5` from a local package cache.
 
     packages:
      "perl5"
      package_policy   =>  "add",
      package_method   =>  smartos_pkg_add("/opt/custom/packages");