Provide JRuby support for model iterating

Includes the following enhancement:
- Loads the correct FFI implementation for your ruby engine.
- Initialize the librdf_world pointer at require time to avoid
  thread-safey issues during lazy initialization.
- When JRuby, disable raptor's URI interning feature because it is not
  thread-safe. This was accomplished by calling raptor functions to
  configure a raptor_world without the uri interning feature.

Issues with these enhancements:
- Requires the caller to create a Redlander::Model per thread to avoid
  additional threading issues encountered in librdf (related to
  librdf_free_stream and librdf_node_normalize).
- JRuby tests segfault related to librdf_parser pointers.