Make sbt download an atomic operation

Modifies the `sbt/sbt` script to gracefully recover when a previous invocation died in the middle of downloading the SBT jar.

Author: Jey Kottalam <jey@cs.berkeley.edu>

== Merge branch commits ==

commit 6c600eb434a2f3e7d70b67831aeebde9b5c0f43b
Author: Jey Kottalam <jey@cs.berkeley.edu>
Date:   Fri Jan 17 10:43:54 2014 -0800

    Make sbt download an atomic operation