0.4.0 release

[bvssvni]

• [Breaking Change] Replace libc::c_void with () for function pointer imports #364 replaced libc::c_void with (), but this has not been published yet
• There are some broken examples Update the gl/examples to build on latest rust and glfw #339
• @tomaka suggested a stable API for gl_generator in Proposition for a stable API #338

[bvssvni]

We might want to implement the changes in gl_generator first, to avoid breaking Glium later.

The "struct_triangle" example could be removed, unless there is a way to use a build script in the examples.

[brendanzab]

Sorry for the delay - I'll get on with this tonight.

[tomaka]

Let's maybe change *const () with *const std::os::raw::c_void again? (#364 in another way)

[brendanzab]

Yes, I would prefer that.