Adding dom helper methods to PropertyPresenter

I want a uniform way of updating the attributes for presentation.

Related to #20
Adjusting logic for label rendering

I want to bring a required class to the label property. This is a step
towards doing this.

Relates to #20
Adding 'required' class to property label attrs

Related to #21
Factoring towards dom_id for label
Merge pull request #28 from ndlib/issues-20-adding-dom-helper

Issues 20 adding dom helper