Update fixtures to not have '/documents/' location
Sets the mongodb up for tests in TestWithMongo
Adds a `StoreProxy` to the `Document`, so you can access the data in the backend

This is supposed to make it easier to get analysis data for a given `Document`.
It also concentrates the code that is dependent on the store implementation in
one place, that should be easily modified if necessary.
Turns `StoreProxy` into a subclass of `dict`

This is necessary so `rest_framework.serializers.Field` will look up attributes
using `[]` (https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/fields.py#L44-45).
Moves view tests to their own package
Filters querysets in all views

This means we return 404 instead of 403 for objects the user has no access for.
We also don't need the 'IsOwner' permission set anymore, since the queryset is
filtered based on the user making the request.
Adds basic plain text visualization for documents
Adds freqdist visualization
Merge the two 'visualization' views into a generic property detail view

I kept the tests as they were in order to guarantee that all previously tested
use cases kept working. This will change soon to avoid duplicate tests.

So, to consider this feature complete we still need to:
    - Have a property list view.
    - Return 404 when trying to access a non-existing property.
    - Merge the two existing tests into one generic TestCase.
Unifies PropertyDetail tests into one generic test case
Handle requests for properties that do not exist for given document
Adds a api call that returns all the properties for a given document
Returns fully qualified URLs in the property-list serializer