Added a new test class to test sub-classed nodes.
Refactored the TreeNode#each method to use iteration instead of recursion.

This will help with trees which have significant depth.
Minor typo fix in the documentation for Tree.
Additional attributions to code contributors.  Updated copyright year.
Updated plans for R0.9.0 and updated version number to 0.9.0.
Added a simple test to check for support for Fibers in the test case.
Added a workaround for running Rake under Ruby 1.9.2.