Make Genson be OSGi-compliant

[lennartj]

In the case of Genson, OSGi-compliance does not require any code changes, but only the addition of a single plugin which synthesizes the MANIFEST.MF files within the artifact JARs.

This yields no problem for the genson artifacts/projects but simplifies life drastically for folks attempting to use Genson in an OSGi environment.

[martin-petzold]

Should be re-opened because the build does not contain OSGi headers!

[lennartj]

Does the felix-maven-plugin execution not fire in the build?

[martin-petzold]

I don't know, but the JAR does not have any OSGi headers or any other OSGi related data.