Assert that OpenGL function pointers are loaded

Closes #59.

Coincidentally, this *should* cover the case where OpenGL was improperly loaded and the software fallback was returned, since the fallback is often the pre-2.0 fixed pipeline and will have none of the functions that should have actually been loaded. According to the specs, `glEnable` should be available in OpenGL >= 2.0 but not prior to that.

I went the little extra mile to make sure the panic message looks nice in a terminal, including some newlines and whitespace to make it easily readable, and kept the lines under 80 columns:
```
task '<main>' panicked at '
    OpenGL function pointers must be loaded before creating the `Gl` backend!
    For more info, see the following issue on GitHub:
    https://github.com/PistonDevelopers/opengl_graphics/issues/103
', src/gl_back_end.rs:322
```
Gnome-Terminal hotlinks the URL to #103, which is really neat. Otherwise it's pretty easily copyable.

The unit test I included is a little superfluous, it just makes sure the assert hits when the functions aren't loaded. I can remove it if need be. It kind of seems out of place, since so many Piston repos forego any kind of unit testing.

I have tested this with `sdl2_window`, and the assert passes.