Bug fixes and workflow improvements

Changes

• Update website
• Bug fixes
• Add R-Datalab Dockerfile example
• Let Rstudio users be added with staff rights so they can install packages etc.
• Add ability to specify disk size when creating a VM (#38) - thanks @jburos
• Add firewall functions (#34)
• Add global operation class
• Add open_webports argument to gce_vm that will open web ports 80 and 443 if necessary
• Add GPU functions
• Migrate to use system2 instead of system for cross-platform SSH (#35)
• gce_shiny_addapp is now much more useful
• Add gce_schedule_docker and gce_vm_scheduler for easy Dockerfile scheduling
• Add gce_vm_logs to quickly browse to an instance logs online
• Fix custom machine types creation (#63) - thanks @Blaza
• Set environment vars on VMs from metadata via gce_metadata_env