Updates to JavaMetabaseGenerator

- exclude com.oracle.*, org.json.*, jdk.internal.*, org.jcp.*, org.ietf.* classes
- Modify code to use a JSONWriter to stream the JSON out as we go rather than build up a huge JSONObject model. This should help avoid using up tons of RAM and out of memory errors.
- Modify the JSON structure for class methods. A method is keyed by it's name, but it's value is an array of objects, each one representing a different signature of that method name. This should allow us to store overloaded methods with different signatures but the same name, rather than clobbering them so last one we encounter wins. Fixes #334