Added pip_install plugin

configuration is like

"pip_install": {"packages": ["a_package", "another_package==1.0.1"]}

Installs build-essential and python-dev
Merge pull request #106 from Jimdo/pip_install_squashed

pip_install plugin
Update volume image path after move it

The `loopback.MoveImage` task wasn't updating `info.volume.image_path`
after moving the volume image file. That made `volume.Delete` to fail
when called, because it would try to remove a file that didn't exist
anymore.
Add `volume.Delete` to GCE taskset

This closes #97.