Add Rails 4.2 compatibility

[andypike]

I needed to add a few methods to allow this gem to work with Rails 4.2
and simple_form 3.1. These methods were as follows:

Attribute#number? - returns true if the attribute type is numerical,
otherwise returns false.

Attribute#limit - returns the database column size limit which
simple_form uses to set maxlength/size attributes on the input controls
if there are no length validations. I return nil here so no html
attributes are added. If you add length validations then these do still
result in html attributes for length being added.

SimpleFormObject#has_attribute? - returns true if the passed attribute
has been declared for the form object otherwise returns false.

In addition to these changes I have also added a new class method for
overriding the model name used (for routing). Now you can do this:

class FooForm
  include SimpleFormObject

  route_as :bar
end

This will result in the form using the path helpers bars_path rather
than foos_path etc.