Add module for SmartOS imgadm command

'imgadm' is a SmartOS command for managing virtual machines images. This first
version provides the following functions :

* imgadm.version()

* imgadm.avail()

* imgadm.show(uuid)

* imgadm.import_image(uuid)

* imgadm.list_installed()

* imgadm.get(uuid)

* imgadm.delete(uuid)