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.
Change the hidden filter into a test

Using a test makes the semantics a little clearer.
Also, testing the widget is a little more precise
than testing the field.
Add Jinja2 test for buttons and a macro for form actions

Also changed hidden loops in bootstrap_wtf to use
hidden as a filtering test.
Have all WTForms imports specify a module

Trying to make it easier to understand which class
comes from where.
Add support for placeholder on types that support it

Also add an example of it in the sample.
Fix the HTML5 widgets based on Input

input != input_type
Factor out error-or-help message to a macro

Will be used in upcoming change.
Support other form layouts on quick_form

Also add an example of an inline form.
Add required in widgets, add more HTML5 widgets

Also removed now-obsolete line in inline_field macro.
Organize widgets and fields into separate modules

There was also some changes, mainly in proper
separation of classes/mixins.