Implement deconstruct() for Django 1.7 migrations support

See https://docs.djangoproject.com/en/dev/releases/1.7/#deconstruct-and-serializability

An alternative implementation would be to use the `deconstructible` decorator from `django.utils.deconstruct` on the field.

Without this, `manage.py syncdb` fails with

    TypeError: Couldn't reconstruct field foo on bar.baz: __init__() takes at least 2 arguments (2 given)