Simplestmaps

- Added support for element sets with the new helpers: marker_set, dot_set, label_set, html_set,
line_set, area_set and geojson_set.
- New and improved docs and examples.

- Hotfix: support for custom coordinates objects with known attributes was broken.

- Hotfix: package description was broken in PyPI.

- Initial version! Markers, dots, labels, htmls, areas, lines and geojsons supported.