Brenda 0.5.1

* Added support for the Blender project files to be made available
  to the render farm instances via an EBS snapshot (instead of S3).
  EBS snapshots can scale up to 1 TB in size.  See README for more
  info.

* When terminating render farm instances that were instantiated by
  a persistent spot instance request, cancel the spot instance
  request.

* Changed default instance type to m3.xlarge

* Updated README