Support map types in swagger generator

[t-yuki]

According to Swagger Schema Object spec http://swagger.io/specification/#schemaObject and proto3 json spec https://developers.google.com/protocol-buffers/docs/proto3#json , map<string, v> field should be represented by additionalProperties.

Internally, protoc generates a entry message type message MapFieldEntry {} from map<KeyType, ValueType> map_field = 1;.
So in this change, mapEntry messages are skipped in message render step and mapEntry fields are expanded as type: object and additionalProperties parameter.
See: https://github.com/golang/protobuf/blob/001690d39bd620847bb265d93a7c5e1bd3737308/protoc-gen-go/descriptor/descriptor.pb.go#L1123

Refs #140

Note that non-string keyed map is not supported yet even though the proto3 json spec said it will be converted to string automatically.
Note 2: generated Go struct field is already map[string]*Object type so there is no fix for grpc-gateway itself and jsonpb is not required. see: https://developers.google.com/protocol-buffers/docs/reference/go-generated#map