Custom inputs namespaces

This add the custom inputs namespace feature that was originally created in this PR #1081.

This PR fixes the custom input class lookup on previous versions of ruby. It also make the custom inputs namespaces has precedence  to top level namespace inputs.

@rafaelfranca Could you take a look?

Thank you @vala for the contributing. 
`constantize` will raise `NameError`, `safe_constantize` would return `nil`.