tvec2, tvec3 and tvec4 static constants.

This pull request is a proposed implementation to https://github.com/g-truc/glm/issues/21. I had actually also looked for something like this in the past so I figured it might be worthwhile to implement it.

The way I've done it is the most intuitive way for me, but I'm not sure if that's necessarily the best way to do it in the context of GLM. The way I've done it is pretty much the same as they do it in Ogre and Unity.