Added simple STL exporter

Adding simple STL exporer. The format only allows geometry, but no textures, materials, nor light. The format also enforces duplicated vertices. It's quite common and it can come in handy.

I also updated the Export example to try it out. Screenshot of the resulting STL file loaded in MeshLab below.

![2014-11-26 16_15_54-multiviewer_containerclasswindow](https://cloud.githubusercontent.com/assets/655605/5203565/189c1890-7588-11e4-8110-ede054ec2c2a.png)

The `Exporter<>` base class did not really suite the STL format as geometry needs to be flat when exported, but I adapted as far as possible to the `Exporter<>` base class.