Generate OSGi headers in the MANIFEST.MF

In order to use this library in an OSGi context, the imported and exported packages must be explicitly defined in the manifest. This pullrequest adds automatic generation of the OSGi import and export headers. It exposes only the 'public' com.lambdaworks.crypto package as API to other OSGi bundles.

Note: I could not get the signing to work locally. Please verify that the signatures are still present in the manifest that is ultimately generated with both signing and OSGi header generation enabled.