Use /rgl instead of rgl/ to ignore only the top-level /rgl directory.

Extract GraphIterator, GraphVisitor, and GraphWrapper from traversal.rb
into separate files so they can be used elsewhere without requiring
traversal feature.

Fix requires in the source and test files.

Add script/test_all that run all tests together and then one by one (to
detect missing requires in the test).

Requiring 'rubygems' inside a gem is considered to be a bad practice.
Here's why: http://tomayko.com/writings/require-rubygems-antipattern.

* Make def_event_handler available wherever GraphVisitor is included.
* Accept symbols in def_event_handler.
* Extract distance map support into a module.

JRuby support is tricky, because of the dependency on 'algorithms'
gem that includes C extensions and the fact that building native
extensions for JRuby is not recommended and most likely will become
impossible in the future.