Point JSONField introspection rule to correct path

This bug was discovered when trying to create a migration on the new exporter django app for a model that had a ForeignKey reference to instance. The following error message was printed:

(env)vagrant@precise64:/usr/local/otm/app/opentreemap$ python manage.py schemamigrati
on exporter --initial
Creating __init__.py in '/usr/local/otm/app/opentreemap/exporter/migrations'...
 ! Cannot freeze field 'treemap.instance.config'
 ! (this field has class treemap.json_field.JSONField)

 ! South cannot introspect some fields; this is probably because they are custom
 ! fields. If they worked in 0.6 or below, this is because we have removed the
 ! models parser (it often broke things).
 ! To fix this, read http://south.aeracode.org/wiki/MyFieldsDontWork

It looks like this was introduced when the JSONField class was moved to its own file, breaking the filepath reference in the introspection rule.
Modify default QuerySet to support auth filtering
Add csv exporter to project

partially addresses #346 on github.
Merge pull request #406 from steventlamb/topic/add_csv_exports

Topic/add csv exports