Recursive data-structure validation with datatype-0.9a3

recursive.py

from datatype.validation import is_valid, failures
from datatype import named, reference

# note the new 
datatype = named('child', {'a': 'str', 'optional b': reference('child')})

# valid
assert is_valid(datatype, {'a': 'some', 'b': {'a': 'foo'}})
assert is_valid(datatype,
        {'a': 'foo', 'b': {'a': 'bar', 'b': {'a': 'baz'}}})

# invalid
assert not is_valid(datatype, {'b': {'a': 'foobar'}})
print failures(datatype, {'b': {'a': 'foobar'}})