Rails 4 support

* tests against not-yet-released versions of dependent gems (formtastic, inherited_reosurces, etc.)
* updates use of `ActiveModel::Name`
* fixes duplicate root route error when run from the root namespace
* includes the appropriate Sass helper depending on Rails version
* update asset pipeline check
use the previous incarnation of #input_class for FormBuilder

For some reason, Formtastic checks if a constant is defined first, which doesn't play nicely with autoloading of custom inputs. This brings Formtastic's implementation back to pre justinfrench/formtastic#783
bump to Rails 4.0.0rc2