Added links to AppScale vagrant boxes

AppScale is an open source implementation of the Google App Engine APIs. For each of our releases, we release a VirtualBox image that can be imported and used with vagrant. This pull request adds links to the two most recently released VirtualBox images (in .box format). These images are pre-built from a base Lucid 64-bit image, and come with HBase, Hypertable, and Cassandra pre-installed.