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?
There might also be other ways to construct an object (through OpenGL extensions or whatever), so there should always be a way to create an object without storage and have access to its id.

Although I agree that the current style is not optimal, I do think that a very thin wrapper is still nice to have (where thin means that we should preserve much of the opengl creation proces, and only take care of cleaning stuff up).

My aim was to address this issue by defining creation functions (https://github.com/m-ou-se/moggle/issues/15). Maybe we can have bare wrappers and nice wrappers having these functions as ctors?