Make constructor ParameterizedType(Type) public

When implementing custom converter of Retrofit, I found LoganSquare.mapperFor(ParameterizedType) is really useful, however, constructor ParameterizedType(Type) isn't public so it's hard to create ParameterizedType.

Example:

``` java
public class ObjectParseConverter implements Converter {
private final Type type;

public ObjectParseConverter(final Type type) {
this.type = type;
}

@Override
public Object convert(final ResponseBody value) throws IOException {
return LoganSquare.parse(value.byteStream(), new ParameterizedType(type));
}
}
```

Start by locating the ParameterizedType(Type) constructor referenced in the issue and inspect its visibility. Expose that constructor publicly, then verify the Retrofit converter example can instantiate ParameterizedType and run the relevant project tests.

Written by the indexing model from the issue text.