[2012] Get `human_attribute_name` working for all inputs

For #2012

For AA, Formtastic's method needs to be changed so that it checks `.base`. I noticed that `FilterBase` was already doing this, so I moved it out to directly monkeypatch `Formtastic::Inputs::Base` so that all input types respect `.base`.

Since this is a direct and global change to Formtastic itself, I created a new directory structure: `lib/formtastic/`. I'm welcome to other ideas.

Unfortunately, it doesn't work just like that. I'm getting this error:
```
Formtastic::UnknownInputError - Formtastic::UnknownInputError:
  /home/sean/code/gems/active_admin/lib/active_admin/form_builder.rb:161:in `rescue in input_class_by_trying'
  /home/sean/code/gems/active_admin/lib/active_admin/form_builder.rb:149:in `input_class_by_trying'
  (gem) formtastic-2.2.1/lib/formtastic/helpers/input_helper.rb:323:in `input_class'
  (gem) formtastic-2.2.1/lib/formtastic/helpers/input_helper.rb:238:in `input'
  /home/sean/code/gems/active_admin/lib/active_admin/form_builder.rb:19:in `block in input'
  /home/sean/code/gems/active_admin/lib/active_admin/form_builder.rb:176:in `with_new_form_buffer'
```

My guess is this change affects/prevents the input types from being loaded. Any thoughts on this @macfanatic?