1.0.0-rc.1

This release has been inspired by Problem Details for HTTP APIs - RFC7807 and
blog post Structuring validation errors in REST APIs
written by @k3nn7.

The main idea has been to simplify validation process on the client side by flattening errors output. To achieve such
a goal, the whole validation process has been rewritten (and luckily for us, much simplified).

• Changed: Positional validation errors for lists
• Changed: ImageField and FileField requires Data URI
  (issue Raise ValidationError in invalid Data URI by default)
• Removed: Form.fill() method replaced by Form.populate()
• Removed: fill_ methods replaced by population strategies