More precise *Field arguments and instance attributes (#62)

Field constructors are quite inconsistent in Django REST Framework: some
take position arguments, some don't. Some pass positional arguments to
their parent class, others pass only keyword arguments. It took a while
but I sorted through all the constructors and nailed down the precise
behavior.

The stubs did not previously denote keyword-only arguments, and the
order of positional arguments was slightly wrong in many places.

Also, added many instance attributes of *Field classes that were
missing.

Author: intgr