Move JNLP generation logic into a Packager Bundler

Currently the JNLP generation logic live in PackagerLib and always fires whenever a native installer is created.

Move the JNLP logic into it's own bundler so it can be executed independently.

Design question: do we continue to automatically generate JNLP whenever a native installer is created or do we expect manual configuration?

Design Answer: For JDK8 JNLP will always be generated when javapackager is run, wether or not a -native option was passed in.

For JDK 9 this will not be the case, and specifying any specific bundler will result in no JNLP being generated, unless the 'jnlp' bundler is specified.

So JDK 8u60 should result in the same outputs as 8u40.