Fix #4 : method root added inside navigation_extensions

	modified:   ../lib/rgen/metamodel_builder.rb
	new file:   ../lib/rgen/metamodel_builder/navigation_extensions.rb
	new file:   metamodel_builder_navigation_extensions_test.rb
Fix #3: implemented methods children, children_deep and traverse in navigation extensions

	modified:   ../lib/rgen/metamodel_builder/navigation_extensions.rb
	modified:   metamodel_builder_navigation_extensions_test.rb
Working on same_content? method

	modified:   ../lib/rgen/metamodel_builder/navigation_extensions.rb
Initial implementation of build_from_value
Fix #1 : Testing method build_from_value
Creating comparison extension
Fix #2 : Testing comparison extensions
	modified:   lib/rgen/metamodel_builder/comparison_extensions.rb
	modified:   test/metamodel_builder_comparison_extensions_test.rb