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?
Thinking about this a bit more, there is a bug in my implementation. If you attach two textures on the same slot (e.g. ```GL_COLOR_ATTACHMENT0```) the former is of course detached, so instead of having a ```vector``` of textures, it should be a ```map<GLenum, texture>```, this way the texture is timely destroyed.