Additions to ext type support for JRuby

[iconara]

I had a hard time explaining what I though must be changed in #91 so I made the changes myself.

• Setters have been removed and replaced with constructor arguments. Adding setters introduces a possibility to change the internals of the objects at any time, something that they don't really support. Since there is no need to change these objects after creation there shouldn't be a way to do it.
• Replace the two extension registries with a single registry. I think this is a design mistake in the C implementation that spills over into everything else. I don't see any point in having separate registries. It complicates everything. The new registry also uses Java collections internally, because using Ruby collections in Java code makes the code very hard to read. The new implementation encapsulates things properly so that the actual implementation is not spread out over three or four different classes.
• I've also removed superflous comments and simplified code.

Unfortunately the tests don't pass in this branch, Factory#registered_types_internal, Packer#registered_types_internal and Unpacker#registered_types_internal have inconsistent interfaces. The consumers of these methods expect slightly different things in the returned values. This seems to be a consequence of the aforementioned design decision in the C implementation, and I consider it a bug. I can't fix it without changing the C code and the design, something I'm not comfortable doing.

@tagomoris this is how I think the JRuby implementation should work, but as I mentioned it requires changes to the C implementation. It's up to you to decide how to proceed.