Wagtail-sharing

- Simplified CHANGELOG format.
- Removed superfluous tests of `Page.get_admin_display_title`.
- Add additional tests for the `before_serve_shared_page` and `after_serve_shared_page` hooks.
- Added Django check and note in README to clarify dependency on `wagtail.contrib.modeladmin` app.
- Fixed MANIFEST.in to properly include only appropriate HTML templates.
- Added unit test against requests without `SERVER_PORT`.
- Added support for Django 2.0 and Wagtail 2.0.

- fix setup.py Django/Wagtail requirements
- Fix minor issues with Django dependencies 21

- Add testing for Django 1.11 and Wagtail 1.13 19
- Add additional hooks to support custom serve logic 20

- Update wagtail dependency 16
- Add trove classifiers for PyPI 17

- Properly limit sharing logic to HTTP GET requests.
- Use simpler method to insert preview banner.

- Convert README to RST for PyPI.