adds sbtApiVersion and scalaApiVersion

sbtApiVersion and scalaApiVersion each calculates the API version of
release builds as well as milestones and RCs.

scalaBinaryVersion and sbtBinaryVersion are now aware of
scalaApiVersion and sbtApiVersion respectively.
For example, sbtBinaryVersion for "0.13.0-SNAPSHOT" now evaluates to
"0.13.0-SNAPSHOT" instead of "0.13".