Add More information to the creation response.

Because it's very useful to know where the object you just created can be
found, we now add the location of the newly created object to the response
JSON. Furthermore, we also add the pk of it to the JSON because that is useful
for libraries that do their own URL construction.
Add support for adding partial objects.

Turns out that a modelform still sets fields with defaults set to required.
This patch fills in the missing defaults.
Added creation tests.

Added four creation tests:
    - Try to create an object with full data.
    - Try to create an object with partial data.
    - Try to create an object with incomplete data.
    - Try to create an object with a wrong body.