Adds new functionality to Schematics models

Schematics has a really useful get_mock_object function.

However, for models which contain ListType and ModelType fields, the behavior is broken

> Household.get_mock_object().validate()
*** ModelValidationError: {'members': [u'This field is required.']}`

If a field is ListType or ModelType, get_mock_object sets the field to None.

MockableListType and MockableModelType inherit from the Schematics ListType and ModelType, but have the correct behavior for get_mock_object():

# this will not raise an AttributeError
Household.get_mock_object().validate()

It's easy to drop the mockable versions in as replacements:

from schematics_extensions import MockableModelType as ModelType
from schematics_extensions import MockableListType as ListType

A subclass of SchematicsModel which raises exceptions if you try to assign a new value to a field. Because sometimes you want immutable values.

> x = Household.get_mock_object()
> x.income = 4000
*** ImmutabilityError: Could not assign value 'income' to 4000 on <Household: Household object>because the object is immutable

A new type which is only valid if the value is a string of decimal digits.

• Add get_null_object to return null-types.