Fix #219: No bridges (and refl proxies) for macros
Fix dependencies of scalajs-library.

It used to depend artifially on scala-reflect.jar and
scala-compiler.jar, because I was still an sbt noob when
I wrote the first versions of external compilation. Now
I know better, and this commit cleans up the dependency
management of external compilation.
Remove 'extends NotNull' for 'js.Undefined'.

The NotNull trait is deprecated as of Scala 2.11.
Merge pull request #246 from sjrd/fix-dependencies

Fix dependencies of scalajs-library.
Merge pull request #247 from sjrd/unify-library

Remove 'extends NotNull' for 'js.Undefined'.
Merge pull request #248 from gzm0/no-macro-bridge

Fix #219: No bridges (and refl proxies) for macros
Compile scalalib overrides with the scalalib, not the library.

This way, their .class files do not pollute scalajs-library.jar.
It also simply makes more sense.