First draft GCE support.

Written by looking at the API docs, but not tested or anything
since GCE is not open to the general public yet and Google won't
reply to my emails :-)
Add support for GCE

Merge branch 'my/gce' of https://github.com/riccardomurri/elasticluster into gce

Conflicts:
	elasticluster/conf.py
PEP8-ification
fix typo