Support missing key-value pairs when creating array

[mrocklin]

It looks like the JSON parser is able to avoid missing key-value pairs for option types

In [3]: nd.parse_json('{x: int, y: ?int}', '{"x": 5}')
Out[3]: 
nd.array([5, NA],
         type="{x: int32, y: ?int32}")

This is great. It would be much greater if it also worked when creating an array from Python objects (this is, at least for me, far more valuable than parsing text).

In [5]: nd.array({'x': 5}, '{x: int, y: ?int}')
BroadcastError: python dict does not contain the field "y" as required by the data type {x: int32, y: ?int32}

While I'm here, it's odd that those two functions have reversed signatures.