Fix #323: AssertionError when running scaladoc.

PrepJSExports referred to phase 'uncurry' for its behavior, but
that phase does not exist under scaladoc. Since the given
behavior is not essential for generating documentation, it is
simply disabled if forScaladoc is true.
Add packageDoc to the Travis build.

So that issues like #323 do not come back haunting us.
Merge pull request #328 from sjrd/fix-publishing

Fix #323: AssertionError when running scaladoc.
The sbt plugin must to be published separately

The sbt plugin is not aggregated by the root project anymore, because
the root project is cross-compilable but the sbt plugin is not. Thus to
publish a local version of scala.js you need to do both:

	> publishLocal

and

	> scalajs-sbt-plugin/publishLocal
Merge pull request #329 from jonas/sbt-publish

The sbt plugin must to be published separately
Fix #330: dce chokes on identifiers with quoted Unicode chars.
Merge pull request #331 from sjrd/fixes-for-dce

Fix #330: dce chokes on identifiers with quoted Unicode chars.