Add automatically generated JPMS module descriptor

[ppkarwasz]

This PR adds configuration to bnd-maven-plugin to generate a module-info.class descriptor.

BND adds the module-info.class descriptor (and META-INF/MANIFEST.MF) directly into the generated JAR and works even on JDK 8 (cf. documentation).

The decompiled module descriptor is:

module [email protected] {
  requires java.base;
  exports com.neilalexander.jnacl.crypto;
  exports com.neilalexander.jnacl;
}

The lack of a stable module name is a (partial) blocker for zeromq/jeromq#591: users that require curve encryption can not (reliably) use JPMS.