Update Capistrano deploy configuration

The first pass just hacked together a code update using the deploy:update task.  This changes that to allow a full ```bundle exec cap staging deploy``` command to run which sets up the releases, current, shared, and repo dirs.  It also runs the custom canvas:compile_assets and takes care of setting up some permissions.  Instructions for prepping the server to handle this are in the "ProductionEnvironmentSetup_Canvas.docx" on Dropbox.