Wagtail-modeltranslation

* Adds support for Wagtail 2.0;
* Create new `migrate_translation` management command that runs migrations and `sync_page_translation_fields`;
* Fix slug auto-population for translation with dash;
* Make localized `StreamField` optional;
* Fix `change_lang` template tag to support multiple parameters;

- added RichTextFieldPanel to the default list of patched panels
- added settings to allow the patching of custom panels
- slug auto-population is now made the same way as wagtail (no changes in live pages)
- Fixed: When adding a page link in a translated RichTextField the link was always to the default language version of that page
- Fixed: Copy content of streamfield fails with 414 Request-URI Too Long
- Fixed: Panel patching failed with the error "AttributeError: 'list' object has no attribute 'children'"

- django-modeltranslation is now a dependency.
- added compatibility with Python 3 (3.3, 3.4, 3.5).
- dropped support for wagtail versions prior to 1.4.
- Fixed: sometimes the required fields weren’t marked as so, raising an Exception not caught on the form.
- Fixed: Patch of panels when a custom edit_handler is defined.
- Fixed: set_url_method which caused to child of a page not being updated when the parent path changed.
- Fixed: Validation of duplicated slugs.