Generate correct types for maps

[stilvoid]

Issue #, if available: #104

Description of changes: Generate correct Go types for maps and undefined types in schema.

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[brianterry]

So, if a user adds a schema without any properties, the generator returns a map[string] TYPE{} instead of a TYPE{}?

[stilvoid]

The metaschema doesn't allow schemas without any properties.

If you define a property like this

{
    "type": "object"
}

you get map[string]interface{} in your model.

If you define the property as

{
    "type": "object",
    "patternProperties": {
        "*": {
            "type": "string"
        }
    }
}

you get map[string]*encoding.String in your model.