Add the faunctionalty of sending a request and have the response in the callback

[osrf-migration]

Original report (archived issue) by Mohamd Ayman (Bitbucket: Mohamd Ayman).

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since gazebojs will mostly be used as a bridge, its kind of a good idea to have sth like this

#!python

boost::shared_ptr<gazebo::msgs::Response> response
          = gazebo::transport::request(name, "heightmap_data");

in it, we actually send requests alots through gazebojs and gazebow.