The from_python of related fields accepts a single value

To be "consistent with the future", the from_python method used by all
the related fields to convert a field/instance to its real value now
accepts a single value, and only one. To use it with many values, simply
use the from_python_many method, or call:

    values = map(self.from_python, values)
ExtendedCollection filter can now accept an instance

In the ExtendedCollection, the _add_filter method (used when creating a
collection) can accept a SingleValueField in addition to a real value.

With this commit, we can now pass a RedisModel too, to use it's primary
key as the filter's value.
Failing tests to show errors in using fields as filter values
Correct passing fields and models as filter value

Before this commit, the name part of the filter wasn't used !
Merge branch 'enhance-collections' into enhance-related