Fix OSGi madness.

The settings must be concatenated in this order,
and `OsgiKeys.exportPackage` must be set correctly,
or we somehow slurp in the whole scala library
into the scala-continuations-library jar
(and scala-compiler into scala-continuations-plugin).
Remove redundant mappings setting.
Merge pull request #2 from adriaanm/osgi

Fix osgi madness