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
Add build.properties, remove from .gitignore
Disable openjdk7 (reduce load). Prep tag-driven releasing.
Upgrade to sbt-osgi 0.7.0

I'm getting bus errors under 0.6.0, which are fixed by
https://github.com/sbt/sbt-osgi/pull/8
Switch to scala-module-plugin, sbt 0.13.1