Replace Object.const_get with constantize in Resourceable module

Object.const_get only works with shallow-nested classes, eg.

    Module::MyResource

whereas constantize work with deeply-nested classes, eg.

    Module::Submodule::MyResource
Only join module and class_name if class_name is implicit. If class_name has
been explicitly use it as-is without modifying.
Cast explicit class_name to_s
Do not nest class in module if module is just Object
Make unless statement easier to read
Add / update deep resource specs
Assert Defining deeply nested resource does not throw a NameError
Ruby 1.8 fix
Version 0.2.1
Fix: Ensure ActiveSupport 3 is used for 1.9.2 support
Version bump to 0.2.2