Brenda 0.5.2

* Set config var AWS_USER="root" by default.  This should
  fix issue where brenda-tool wasn't logging into
  correct account on render farm nodes.

* Added new command "brenda-tool perf" to view render farm
  performance/cost statistics.

* Added new config var DONE (SHUTDOWN now deprecated).
  When task queue is empty:
    DONE=shutdown       -> shutdown instance
    DONE=poll           -> continue polling for new tasks
    DONE=exit (default) -> exit but leave instance running

* Made "brenda-run init" a bit smarter, and will use an existing
  local ssh key pair if one exists, or will otherwise generate
  one on AWS.

* Added "brenda-run reset-keys" to remove ssh key pair and
  security group on AWS.

* Better utilization of EC2 instance store volumes on instances
  that support them.

* Task scripts can now use "-s $START -e $END -j $STEP" instead
  of "$FRAME" (makes it easier to adapt Brenda to parallelize
  other non-Blender applications).