map elements under urdf extension <gazebo> directly under <model>

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/112)) by John Hsu (Bitbucket: [hsu](https://bitbucket.org/%7B0a186eae-abf0-4514-a951-23db5eccc286%7D/), GitHub: [hsu](https://github.com/hsu)).**

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inspired by [this issue](http://answers.gazebosim.org/question/12611/urdf-to-gazebo-differs-from-urdf-to-sdf-to-gazebo/). For example, following `pose` block in urdf:

~~~

0 0 0.5 0 0 0

~~~

will go directly under SDF model as:

~~~

0 0 0.5 0 0 0

~~~