Support dynamically changing mesh type at runtime 

Allows the meshing type to be changed live, for example:

    game.meshType = 'wireFrame'
    game.showAllChunks()

to switch all chunks from surface meshes to wire framing, no reloading needed:

![e19903303b641faced67c456aa5f280c](https://f.cloud.github.com/assets/5897956/1811852/26653f96-6e64-11e3-81d9-bb6ca79b83d1.gif)

Useful for testing different meshing algorithms (game.mesher can already be changed at runtime without issue).