Moved default date/time formats to a formats module and exposed it as forms.formats
Implemented active validation for Fields which uses Input widgets

WIP on #6

New API:

* `validation` and `onStateChange` kwargs for Form
* `validation` kwarg for Field
* validation: null or 'manual' (off), 'auto' (onChange with 250ms delay), a camelCase event name or an object with 'event' (event name) and 'delay' (in ms) properties can be given
* onStateChange: *must* be provided if a form or any of its fields are using active validation, or an Error will be thrown

Widget.render now takes a fourth argument for validation config.
Implemented Input.render as a test of the API.
Dev build