Pform

----------------

- Added `Composite` field

- Render str in `form:error` message

- Removed `display` mode

- SimpleTerm and SimpleVocabulary renamed to Term and Vocabulary

- Removed `from_items` and `from_value` SimpleVocabulary methods,
use Vocabulary constructor instead

- Use `form` layer category for form and field templates

- Button class now inherits from InputField

----------------

- Allow to add `str` as error in `Form.validate` method

- Better button handling

- Better tanslations for `Invalid` messages

- Changed arguments order for `Invalid` class contrustor

- Added sub errors support to `Invalid` class

- Added `extract` parameter to `Button` class, if extract is set then action
extract values from form, adds errors to message and pass data to handler

- Added `pform.button2` decorator, `extract` parameter is true

- Added `max_size` and `allowed_types` parameters to FileField

- Do not override widget's custom tmpl_widget

----------------

- Fixed submit button template

- Allow to return http response from form update or action handler

- Added support for `player.layout()`

----------------

- Initial release