Add SimpleForm.custom_inputs_namespaces config to allow namespacing custom input classes
Use the custom namespace to find the custom input.

In previous ruby versions(1.9.3) we can not use `Object.get_const('CustomNamespace::MyInput')`,
the correct way is `CustomNamespace.get('MyInput')`.
Add precendence to custom inputs on custom namespaces.
Add test to make sure the not maped inputs on custom namespaces has precedence.
Make sure when a not existent namespace is passed raises error
Add some grammar full stops and use the Simple Form name when is not associated to the class
Merge pull request #1093 from plataformatec/custom_inputs_namespaces

Custom inputs namespaces