rdf-gzip: gzip support for rdf.rb readers and writers

Usage:

	require 'rdf'
	require 'rdf/ntriples'
	require 'rdf/json' # or whatever serialization
	require 'rdf/gzip'
	
	graph = RDF::Graph.new
	
	RDF::Reader.for(:ntriples).gzopen("/path/to/rdf.nt.gz") do | reader |
		reader.each_statement do | statement |
			graph << statement
		end
	end
	
	RDF::JSONWriter.gzopen("/path/to/rdf.json.gz") do | writer |
	  graph.each_statement do | statement |
			writer << statement
		end
	end
	
That is to say, exactly like how you would normally user RDF::Reader and RDF::Writer, but use gzopen rather than open.  The format needs to be called explicitly (that is, no file format detection) due to the fact that the file suffix may not exist in the file name anywhere.