Rename HtmlField to PluginHtmlField, fix name inconsistency.

The other model fields also start with Plugin.. so also apply this to
this model.
Make django-any-imagefield + django-any-urlfield optional in the picture plugin

Using the wrappers classes (`PluginImageField` and `PluginUrlField`) instead.
Update documentation for new features in 0.9.0

- picture plugin
- FLUENT_CONTENTS_PLACEHOLDER_CONFIG setting
- custom model fields.