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.
updated ignores
Added a compact measurement serializer

Reduces API response size by about 70%. Having done that, we'll need a
new measurement details API for getting the full info for each one.
Merge pull request #11 from jimr/filters

Added API result filtering based on measurement date
default to compact response for XHR
Merge remote-tracking branch 'jimr/compact'

Conflicts:
	observations/api/views.py