Django-bootstrap3

- Drop support for Django 1.8 (434)
- Fix bug in demo app (430)
- Remove unnecessary `len` call (424)
- Switched to master as main branch, deleted other branches
- Switched to twine for publication on PyPI

- Mention django-bootstrap4 (https://github.com/zostera/django-bootstrap4) in README
- Rewrite `tox` test matrix to focus on Django releases rather than Python versions
- Add tests for Django master branch (>= 2)
- Add `label` override for `{% bootstrap_field %}`

- Renamed requirements-dev.txt back to requirements.txt because that suits ReadTheDocs better
- Added `error_types` support on `bootstrap3_form` (thanks mkoistinen and ickam)
- **BREAKING** Default setting of `error_types` to `non_field_errors` is different from behavior in versions < 9

- Renamed requirements.txt to requirements-dev.txt
- Tweaks to tests and CI (see 400)
- Prepared test for geometry fields (disabled, blocked by Django update, see 392)
- Bug fixes for add ons and placeholders (thanks jaimesanz, cybojenix and marc-gist)
- Improve documentation for pagination with GET parameters (thanks nspo)
- Add unicode test for help_text
- Removed tests for Python 3.2 from tox and Travis CI (no longer supported by Django 1.8)

- Fix invalid HTML in help texts (thanks luksen)
- Added `mark_safe` to placeholder (thanks ppo)
- Fix DateWidget import for newer Django versions (thanks clokep)

- Support for local languages in `url_replace_param` on Python 2 (362, thanks aamalev)
- Correct checking Mapping instance (363, thanks aamalev)
- Fix Django 1.11 import bug (see 369)
- Add Django 1.11 and Python 3.6 to tests
- Fix sdist issue with .pyc files