Factor boilerplate from phantomjs to an sbt-dependent library

[Stephen Compall]

phantomjs, the sample of a capturing library, manages to offload *most* of its build system magic to the core platform-executing library. There's still a bunch of weirdly specific boilerplate, though.

Much of this could go in a p-e dependent, sbt-dependent library.

* unmanagedSources in Compile <+= (PhantomJS.java "generator"): Should be a triple of these properties as sbt settings instead, which generates the relevant Java file.

* phantomjsBinaries, phantomjsVersion: I don't think we can produce those Scala symbols without adding a metametaproject, but the setting keys are straightforward, and you get a nice obvious error when you fail to set them.

* licenses <++=. Supply the licenses present when a binary is present, and it will add an entry when it's right to add.

* mappings in (Compile, packageBin): Pure boilerplate given the above.

[Stephen Compall (scompall)]

phantomjs r53 illustrates usage of the new style.