Add a JSONField that translates db strings to python via JSON

Just make a JSONField field:

```
class MyModel(models.Model):
   stuff = JSONField()

...

m = MyModel()
m.stuff['user'] = {'age': 4, 'name': 'Peggy'}
m.save()

```

JSONField does not currently support lookups.
Add config JSONField to instance

Also provides a way to create simple getters/setters. If we want to add
a new top-level setting simply run:

```
advanced_search_filters = _make_config_property(
   'advanced_search_fields')
```

Which will allow you to write and read that property directly from an
instance:

```
i = Instance.objects.get(pk=?)

i.advanced_search_fields = ['field 1', 'field 2']
i.save()
```
Merge pull request #206 from ahinz/topic/instance-config

Topic/instance config