Added a source bundle for the sbt library.

This commit allows debugging, hyperlinking, etc. towards
sbt sources. Useful when working on the Sbt builder.

* create a source bundle for the sbt library. I needed to
  inline the contents of sbt jars in order to get Eclipse
  to correctly associate sources with classfiles

* created a new compiler.interface bundle, only for the
  compiler interface. Sbt requires this to be a separate jar
  and there was no way to build a single sbt bundle with
  only some of the dependencies inlined.

A similar approach could be used for the Scala library/compiler
bundles, but we should investigate the possibility of having the
Scala team publish source bundles too.

(this was originally a commit from @dragos, but after rebasing against
master it changed owner :))
Some refactoring, but this won't work until https://github.com/typesafehub/sbt-republish/issues/4 is fixed.