Give zinc wrapper ability to create jar files

[illicitonion]

The zinc wrapper we have at https://github.com/pantsbuild/pants/tree/3dc6cce95c5e38c75b77b81ebd4ca01a74dff9ea/src/scala/org/pantsbuild/zinc/compiler currently takes a flag _classesDirectory in Settings.

We should give it a new flag, outputJar. If it is specified, we should jar up the contents of _classesDirectory (using the standard java.util.jar.JarOutputStream, unless there's a good reason not to) and output it to the path pointed to by that flag.

We should use a single, deterministic timestamp for each entry which we create, so that the jarfiles are byte-for-byte reproducible.

Maybe we want to make classesDIrectory an optional flag, such that the valid combinations are:

1. classesDirectory set, outputJar not - the current behaviour.
2. classesDirectory and outputJar both set - classes will be in the former, jar file at the latter
3. outputJar set, classesDirectory not - classes will be created in a tempdir, jar'd, and then deleted.

And an error results if neither are set.