Experiment: Set correct type in constructor

Based on [this discussion](http://www.enrise.com/2010/12/html5-zend-framework-form-elements/#comment-2217) this is a prototype for Glitch_Form_Element_Text_Date.

Currently, even if you create an instance of this class and name it anything but `date`, you have to manually set the type of the element.
This adds a new constructor which sets the type if this has not been done in `$spec` or `$options`.

If this is an acceptable solution I will update the other element types accordingly.