Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.
Deprecate usage of boolean value for widget attributes

Django 1.7 will loudly warn when widget attributes are assigned
boolean values. In Django 1.8, False will mean attribute is not
present while True will mean attribute present without value.
Refs #20684.
Tweaked proj string regex in gis tests
Added release dates for gis libs as doc comments
[gis] Dropped official support for GDAL < 1.6
Fixed #14006 -- Documented that Field's 'description' attribute is interpolated with field.__dict__

Thanks abeld for the suggestion.
Fixed #20711 -- Fixed broken link in timesince.py docstring
Fixed #16284 – simplified language selection in jsi18n.

English is no longer treated as a special case and there is no
fallback to default language code.