Geokey-sapelli

We officially now be supporting only GeoKey v1.6+ because of Django v1.11 support ⭐️

This patch also adds README to [PyPI deployment](https://pypi.org/project/geokey-sapelli/).

BUGFIX: Sapelli projects with optional decision trees can now be uploaded.

Improved and redesigned extension with support for:
- Sapelli forms without locations (set as Null Island, suggested by mukih)
- Sapelli forms with multiple locations (will be merged as one MultiPoint location)
- Upload Sapelli logs
- Filter and download logs individually or as a ZIP batch

**BREAKING CHANGE**

Only one the same Sapelli project is allowed to be added per platform (no duplicates allowed anymore) due to Sapelli project ID & fingerprint issue.

If someone else is adding the project you want to contribute to, make sure the original owner adds the right permissions to your user! Otherwise make sure to change the Sapelli project ID, so that it would be treated as a different instance.

Full stable release.

**BREAKING CHANGE**

Due to changed API in GeoKey v1.0, the geokey-sapelli extension (this and later releases) will no longer support older versions of GeoKey. Also, geokey-sapelli <1.0.0 will not support GeoKey v1.0+.

- Added `accept="..."` attribute to file upload `<input>`s. This should help users to avoid uploading invalid files (e.g. zip instead of csv, or xml instead of sap/zip)
- The extension is now compatible with [Django v1.9.x](https://docs.djangoproject.com/en/1.9/releases/1.9) (thanks juliusosokinas)
- Travis script now runs against both Django v1.8.x and v1.9.x (thanks juliusosokinas)
- Simplified version definition (back in line with other extensions)
- Note: requires [GeoKey v0.9.3](https://github.com/ExCiteS/geokey/releases/tag/0.9.3) or later