Provide JRuby support for model iterating

[abargnesi]

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.

[abargnesi]

This commit enables concurrent access to librdf storage through Redlander::Model. The caller is required to create a Redlander::Model per thread (e.g. Thread.current[:model] ||= Redlander::Model) to avoid segfaults in librdf.

Thoughts?

[cordawyn]

Thank you for the contribution, and sorry for my delay in responding to it.

First, I'm absolutely fine with creating Redlander::Model using Thread.current. I think that's how I do it in kalimba. Or not. But I should have done so anyway ;-) In any case, Rails should be happy with that approach.

However, I'm somewhat concerned with initializing librdf_world at require time. Isn't initializing at class level and storing stuff in a constant (Redlander::RDF_WORLD in our case) the primary cause of "thread-unsafety"? So... librdf_world is going to be shared between threads. Is it immutable? I guess we should check with the C side of librdf, unless you've done so already.

You also do not have a finalizer defined for Redlander::RDF_WORLD. I believe it should be there somewhere.

And 2 more things :-)

1. Could you delete unused code? I'm talking about those mri?, jruby? and rubinius? methods. They're good, but I'd rather avoid stale/unused code. Besides, it's not the competence of Redlander to check Ruby version, if we look at it logically ;-)
2. I'd appreciate it if you used double colons :: to separate namespaces (constants), but use a dot in front of method calls (e.g. Redland::load_raptor -> Redland.load_raptor). Thank you.