Added getPropertiesForGID that assings value instead of returning it.

If you are looping through all the tiles in a tilemap to get their properties. The original getPropertiesForGID  function will create an empty Value object if the tile has no properties at all. This can be really inefficient if you are looping through a map that mostly has tiles with no properties.

The new function will allow you to skip the empty object creation (this saves a lot of time) giving you the option to send as an argument the variable you want the value to be assing to. An example of how the new function would be use:
```cpp
Value* tilePropertiesValue = NULL;
if (tileMap->getPropertiesForGID(gid, &tilePropertiesValue )) {
    ValueMap tileProperties = tilePropertiesValue->asValueMap();
    //do what you need with the tile properties
} else {
    //handle no properties on tile
}
```