make documetation generate again

* prevents exceptions when generating shell examples
* prevents exception when defining @option on a module
* use latest yard
* replace rdoc in rakefile
change :nodoc: to @private

Conflicts:
	lib/formtastic/helpers/enum.rb
generate doc for configuration options
improve documentation of namespaced class inputs
Use `eager_load` instead of `cache_classes`

In `formtastic/helpers/input_helper.rb`, `eager_load` is used to
determine if `const_defined?` should be used.
With `cache_classes = true`, classes are still not loaded at startup, so
some inputs may not be loaded.
add links to upgrade guide to deprecation notice
add CHANGELOG for 3.1.1