Powershell import (mods)

This is basically my PR to @sempervictus's branch https://github.com/rapid7/metasploit-framework/pull/2075

He seems to have gone awol/quiet for a while and I don't have a way to reach him. Dunno if anyone else is interested in powershell but I've made a number of adjustments to it.

More and more modules are starting to poke at the powershell functionality so I would rather get these in earlier rather than later.