support form input value, removing attributes

I converted a small app from a self-submitting iframe that did all rendering on the server to an Espresso app. (Ended up ~150 lines coffeescript compiled to >300 lines javascript.) I needed the features added here to properly handle form elements, especially removing a disabled attribute once it was added and setting the value of form elements (selects are much easier to set this way, textareas can only be set this way).

I think there's a case to be made for supporting false as well as null for attribute removal, as I can't think of any circumstance in which I would set an attribute to boolean false and *want* the result to be attribute="false", but I didn't make that further change.