Merge pull request #2 from stringfellow/master

Merged changes from upstream.
Add API result filtering based on observation date

Using django-filter (will need a pip install from requirements.txt).

Maps now default to showing data from 2008 onwards, but more as a
demonstration of how to hook the filtering into the maps than as a
particularly useful feature.
Merge pull request #11 from jimr/filters

Added API result filtering based on measurement date