Fix for #54 (for core)

I've got a patch that, for the most part, does away with using the template MANIFEST.MF and fixes #54 in favor of using maven-bundle-plugin which puts the appropriate OSGi metadata in the MANIFEST.MF.

I think that this will need to be applied to the guice and spring integration bits as well, but it's not clear to me how to best handle those in an OSGi environment. Probably would want to build them as fragment bundles so they could share the classloader with core (and thus use the internal classes which are not exposed to the OSGi environment at large).

I've tested this in our own deployment using Apache Karaf (Fuse ESB) and it works, though.