Add Map Support

[benfrew13]

As of version 3.0.0 map fields are valid in protobuf messages. FieldUtils doesn't handle these properly, as it treats them as a standard scalar field and generates the wrong getter/setter names.

Example:

message Test {
    map<int32, string> sample = 1;
}

The correct getter and setter for sample are putAllSample and 'getSampleMap`.

This change appears to be sufficient to get maps with scalar types for keys and values, but protobufs also allows nested complex types for values.

message Test {
    map<int32, Nested> complexField = 1;
}

message Nested {
    string name = 1;
}

It looks like both the default Mapper and Converter would need work to support this new category of fields.

[jsjem]

Good point. I will try to add map fields support.

[djotanov]

I've created PR which adds support for the map type. Could you please review?