Extend Map returns to supports more types than basic and json (enums, etc...)

The following proxy interface doesn't compile:

``` java
package com.melusyn.gatekeeper.service;

import io.vertx.codegen.annotations.ProxyGen;
import io.vertx.codegen.annotations.VertxGen;
import io.vertx.core.AsyncResult;
import io.vertx.core.Handler;

import java.util.Map;

/**
* The service interface.
*/
@ProxyGen
@VertxGen // Generate the proxy and handler
public interface MyProxyService {

enum MyEnum {
Value1,
Value2,
Value3,
}

void callThatReturnsAMap(String param1, Handler>> handler);
}
```

The compilation fails with the error:

```
type >> is not legal for use for a parameter in proxy
```

However, the documentation says otherwise (https://github.com/vert-x3/vertx-codegen#permitted-types).

> The following set R of types are permitted as return types from any API method:
> [...]
> type java.util.List, java.util.Set or java.util.Map where C contains
> [...]

The method `io.vertx.codegen.ProxyModel.isLegalListSetMapResult` does not check for `Map` type.

Has this been overlooked or this is on purpose? I found an old issue #8 referring to it... But not much was said at that time.

I'm willing to do a PR if the core team feel this is missing and should been implemented.