Remove aux components.

These will be moving to the opengl-aux project / gem.
Most of version 2.0.0.

- Gl module is now GL. You can easily undo this by adding 'Gl = GL'
  somewhere in your code, but I figure it's probably saner to use GL at
  this point.

- All aux/ types and functions are removed. These are part of the
  opengl-aux gem, now found at https://github.com/nilium/opengl-aux

  opengl-core is now 100% raw bindings.

- The symbol loader can be swapped out with another, so if Fiddle isn't
  your bag, you can replace it by changing GL::GLSym.loader to a
  different loader class. See the comment on GLSym for more info about
  what a symbol loader must implement and see FiddleSymbolLoader for
  what is the only implementation currently available.

- Using a method that isn't implemented will now raise NoMethodError
  upon attempting to load it. load_all_gl_commands! will swallow this
  exception, though I'm not sure it's a great idea to use that when it
  increases the amount of loaded methods to look up when calling a GL
  function.

- Use - instead of _ in file paths.
Update .gitignore.
Update license.
Update the README.

Having a contributors list with only me in it looks dumb, but maybe
someone with a weird hatred of Fiddle will write a symbol loader using
a different FFI library or fix something and then it won't look so bad.
Remove C extensions. They're unused.
Version 2.0.0

- Removed date (why did I put that in there?).
- Oh, yeah, the C extension is gone now too.
- Update dependencies.