Record Scala version used to build sbt incremental compiler

[dotta]

It would be convenient to have a .properties file injected in each of the Zinc artifacts to know the Scala version it was compiled with. This is useful to make sure we don't attempt to build an IDE that is doomed to have binary incompatibilities.

[dotta]

@jsuereth Is this something you (or someone else in Q branch) would have time to do, or should we have a stub at it? I'm quite paranoid about making sure we use the same Scala compiler to build the whole chain.

[jsuereth]

I don't think this will take long, but I'm not sure any of us who can do it quickly have the time. I'll check with the others.

What we need:

1. Figure out where the block of code which reads the exact scala version out of the jars lives (I think there's something already in sbt)
2. Expose this reading/setting as a task (depends on dependencyClasspath and rips out scala version.
3. Create another task which writes a new properties file with the full scala version in it.
4. Add the propertiesFile creation task to resourceGenerators (I think).
5. Figure out if this behaves with sbt-assembly (our current usage).

I'm currently in "ZOMG RELEASING FIXES" mode for the next 2 weeks.