Provide a default renderer

[jeremyfa]

The runtime is already tested with a closed source renderer (for a closed-source game engine), but a default renderer can be provided to make it easier to integrate with common Haxe frameworks.

The first renderer implemented is likely to be for OpenFL as it is the most popular framework used with Haxe. There are just some questions about deciding which API should be used to render meshes in OpenFL (see http://community.openfl.org/t/trianglemap-renderer/9613)