Updated version requirements for the Gem dependencies.

Also created other minor changes for the Gemfile and bundle configuration.
Changed 'postordered_each' to use iteration.

This change fixes bug #12, where the recursion for deep trees was leading to out
of stack memory errors.
Minor modification to the chmod settings.
Moved the tree metrics methods to a new mixin.
Multiple documentation related changes.

Fixed the link references across the document set.

Also, started using the Yard reference tags to include example code where appropriate.
Added example code to the Gem.
Reverted JSON to 1.7.x
Fixed bug #23 where a nil left_child would cause a crash

This bug was being manifested for BinaryTrees.
Fixed the yield parameter documentation.
Added an inordered_each method for the Binary Tree
Fixed the return values for the iterator methods.
Documentation fixes.
Modified the Enumerator tests to support Ruby1.8 & 2.0
Checking for Enumerable::Enumerator for Ruby 1.8.x
Added the version badge.
Merge branch 'next' into develop