Adds renderbuffer, texture, fbo. Adds get_id() functions.

The renderbuffer and texture wrapper classes are really straightforward and can be pulled right away (I guess).

I needed ```get_id()``` functions on those classes to be able to work with OpenCL. I can image such functionality is also useful when using certain OpenGL extensions (or even functions we do not wrap already).

The ```bare_fbo``` wrapper is also quite easy and lacks (just like ```vao```) the ability to attach temporary textures or renderbuffers. The ```fbo``` on the other hand can attach those (and stores them himself). Maybe such an approach is also possible with vao's?
I'll hop on your train of thought. Shouldn't we then, also in the style of shared_ptr, also have a ctor that takes an id an becomes the owner of the given idea?

The thin wrapper idea is good, providing a minimal RAII wrapper is a nice endeavor. However, rather than the factory functions I'd rather see additional constructors. Agreed?