Added new tech building pixelart contributed by the community

Thanks to @CombineCR, @bellator and @NukemBro. Can be used for map decoration.

![Tank Factory](http://content.open-ra.org/images/screenshots/units_290_10_ex.PNG)

![Allied Techcenter](http://content.open-ra.org/images/screenshots/units_311_13_zz.png)

etc. (click the links for previews)