Add disableValidateReadonly setting

[er314]

Allow to perform validation when field is readonly.

Use case :
When some javascript component actually populates the input field content (not directly the user), for example a javascript date picker.
The field value is GUI-selected by the user via the javascript widget.
The field is readonly, to prevent user from directly inputting textual value in wrong format.
Yet, it is expected that bouncer.js performs validation of the input (and refresh of the error message), on the "required" html constraint ( = the field must have a value).

How :
I propose to add a setting : disableValidateReadonly
when set to true (default), the behavior is unchanged : bouncer.js does not perform validation when a field is readonly.
when set to false, bouncer.js performs validation when a field is readonly.