Fixed choropleth (#1005)

Better handling of binning and color scales in the choropleth method in the Map class.

Add nan_fill_color and nan_fill_opacity arguments.

The `threshold_scale` argument has been deprecated in favor of the new, richer `bins` argument. The old `threshold_scale` argument still works though.

Author: FloChehab

CHANGES.txt

@@ -6,6 +6,11 @@
 - Add MousePosition plugin (btozer #916)
 - Add Minimap plugin (talbertc-usgs #968)
 - Replace Rawgit CDN with Githack (jtbaker #1002)
+- Handling of NaN and missing values in choropleth (FloChehab #1005)
+
+API changes
+
+- `threshold_scale` argument of choropleth is replaced by `bins` (FloChehab #1005)
 
 Bug Fixes