Implement HTML5 widgets and fields

WTForms 1.0.4 should have these included, but it
hasn't been released yet.
Add a Bootstrap-specific checkbox field

This is a weird call, as Bootstrap wants the input
to be within the label element.
WTForms uses unicode internally, so will we
Rewrite BooleanField to be conceptually simpler

It's a lot closer to what the RadioField will be
like.
Add examples of BooleanFields
Add Bootstrap-ready RadioField + Example
Factor out class="…" and blank <label>

Blank <label>s is now done by a DummyLabelMixin,
and the class attribute modification is done by a
common function.
Add Button fields and examples

Two types of Fields are provided, <button> and
<input>. type="reset" aren't provided, but can be
provided by subclassing ButtonField or
ButtonInputField.