Add sbt-buildinfo

[ashawley]

http://github.com/sbt/sbt-buildinfo

After ScalaJS is merged, add to plugins.sbt and build.sbt file. There should be a way to verify what version of this library is on the classpath.

In particular, adding the library version to the namespace will be useful for debugging scala/scala-dev#291

[ashawley]

The scaladoc dependency issue is better understood than I had thought. This isn't as necessary or beneficial as I thought it would be.