Get Ruby bindings to work with Tree class

[jbcoe]

Needs:

• [x] Support for void return type
• [ ] Support for sub-object return type
• [x] Support for method arguments
• [x] Support for constructors of base classes
• [ ] Running multiple Ruby tests from CMake
• [x] A TestTree.rb file

[jbcoe]

Work in progress in branch n140-enhanced-ruby-bindings