JSON Schema for a simple JSON is not supported #58

schema={
  "$schema": "http://json-schema.org/draft-04/schema#",
  "title": "Example",
  "type": "string",
  "enum": ['abc', 'xyz']
}

>>> import warlock
>>> Example=warlock.model_factory(schema)
>>> Example('xyz')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/warlock/core.py", line 35, in __init__
    base_class.__init__(self, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/warlock/model.py", line 30, in __init__
    d = dict(*args, **kwargs)
ValueError: dictionary update sequence element #0 has length 1; 2 is required

>>> Example(_='xyz')
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/warlock/model.py", line 140, in validate
    jsonschema.validate(obj, self.schema)
  File "/usr/local/lib/python3.6/site-packages/jsonschema/validators.py", line 934, in validate
    raise error
jsonschema.exceptions.ValidationError: {'_': 'xyz'} is not of type 'string'

Failed validating 'type' in schema:
    {'$schema': 'http://json-schema.org/draft-04/schema#',
     'enum': ['abc', 'xyz'],
     'title': 'Example',
     'type': 'string'}

On instance:
    {'_': 'xyz'}

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/warlock/model.py", line 33, in __init__
    self.validate(d)
  File "/usr/local/lib/python3.6/site-packages/warlock/model.py", line 142, in validate
    raise exceptions.ValidationError(str(exc))
warlock.exceptions.ValidationError: {'_': 'xyz'} is not of type 'string'

Failed validating 'type' in schema:
    {'$schema': 'http://json-schema.org/draft-04/schema#',
     'enum': ['abc', 'xyz'],
     'title': 'Example',
     'type': 'string'}

On instance:
    {'_': 'xyz'}

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/warlock/core.py", line 35, in __init__
    base_class.__init__(self, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/warlock/model.py", line 35, in __init__
    raise ValueError(str(exc))
ValueError: {'_': 'xyz'} is not of type 'string'

Failed validating 'type' in schema:
    {'$schema': 'http://json-schema.org/draft-04/schema#',
     'enum': ['abc', 'xyz'],
     'title': 'Example',
     'type': 'string'}

On instance:
    {'_': 'xyz'}