Make jzlib an optional dependency (OSGi)

[lichtin]

Can jzlib be made an optional dependency?
Currently it is required

Caused by: org.osgi.framework.BundleException: Unresolved constraint in bundle net.schmizz.sshj [160]: Unable to resolve 160.0: missing requirement [160.0] osgi.wiring.package; (&(osgi.wiring.package=com.jcraft.jzlib)(version>=1.1.0)(!(version>=2.0.0)))

I believe the change needed is in pom.xml:

                    <Import-Package>
                        !net.schmizz.*,
                        javax.crypto*,
                        com.jcraft.jzlib*;version="[1.1,2)";resolution:=optional,
                        org.slf4j*;version="[1.7,5)",
                        org.bouncycastle*,
                        *
                    </Import-Package>

Perhaps the same should be done for bouncycastle.