Add OSGi manifest so that Spectrum can be used in OSGi projects

[raner]

I'd like to use Spectrum in an OSGi-based project, but the JAR does currently not have an OSGi manifest. This can be easily added either in a hard-coded fashion by adding a src/main/resources/META-INF/MANIFEST.MF file, or by adding a build step to the Gradle build file that generates a manifest automatically.
If there is a chance that such a change could get released to Maven Central soon, I'm happy to make a pull request with the necessary changes.