Second pass at handling post meta

[rmccue]

Based on #168.

Still not at a point where I like it. We're now outputting:

[
    {
        "key": "my_test_field",
        "value": [
            "Some value!"
        ]
    }
]

Whereas the input is:

[
    {
        "key": "my_test_field",
        "value": "Some value!",
        "action": "update"
    }
]

[rmccue]

@rachelbaker @tlovett1 Thoughts?

[rmccue]

Obsoleted by #189.