Update to sbt 0.13.0.

The build.properties file mentions that 0.13.0 breaks plugin
resolution but doesn't mention what plugins are broken. I tried upgrading
and everything works fine so I'm committing that change.
Configure credentials file for publishing.

Credentials file is required for publishing artifacts. Standard
configuration taken from:
http://www.scala-sbt.org/release/docs/Detailed-Topics/Publishing.html#credentials