This adds methods to the Jar class to read versioned content of a
multi-release jar for the following cases:

- get a resource or a map of resources for a given release version
- get a merged manifest according to the OSGi specification
- get module name/version for a given given release version
- list all contained release alternative versions

Beside this a unit-test is added to cover these new functions.

Signed-off-by: Christoph Läubrich <laeubi@laeubi-soft.de>

This changes enable BND user to set

Multi-Release: true

in a BND file and then creates a bundle that conforms to the OSGi
Multi-release JAR specification

Signed-off-by: Christoph Läubrich <laeubi@laeubi-soft.de>