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 still think it'd be nicer to have it have shared_ptrs to textures. Why would we burden the users with managing lifetime while it can be perfectly managed by shared_ptr? That was the whole idea behind it. You know how much of an annoyance it is to deal with this yourself in more than toy-example programs.