Add Google+ Auto Backup.app latest version

This is the backup tool that comes with latest version of Picasa.
Add it as its own cask if you don't want to download Picasa.

More info:
https://support.google.com/picasa/answer/4392268?p=gpautobackup&rd=1

Add Picasa as Homepage

The Google+ Auto Backup comes as part of Picasa and so this homepage
makes the most contextual sense, even though you don't have to install
Picasa to use it.
Merge branch 'google-auto-backup' of github.com:samtsai/homebrew-cask into google-auto-backup

* 'google-auto-backup' of github.com:samtsai/homebrew-cask:
  Add Picasa as Homepage
  Add Google+ Auto Backup.app latest version