Refactoring of the build process to publish artifacts.

The build process is much cleaner now. And it can
produce and publish artifacts for the compiler and the
Scala.js library.
Use `compile` instead of `compileJS` to compile JS files.

This further simplifies the build script.
Use classDirectory setting in build script.
Use compileInputs settings in compile task.
Separate boot cp from normal cp.
Make javalib depend on library.

Instead of adding the library sources to javalib as a hack.
Added the Compile . Scala.js sources... info.
Extracted an sbt plugin for Scala.js projects.
Bump sbt version to 0.12.3.
Updated the readme.