Overhaul GValue bindings

[gkoz]

Here's the best compromise between type safety and ergonomics I've managed to achieve.
cc #73

Examples:

    let mut value = glib::Value::from(Some("Hello world !"));
    println!("gvalue.get example: {}", value.typed::<String>().unwrap().get().unwrap());

    let mut int = glib::Value::from(1);
    int.typed_mut::<i32>().unwrap().set_some(&2);
    assert_eq!(int.get(), Some(2));

    for i in 0..10 {
        let iter = right_store.append(None);
        right_store.set_value(&iter, 0, &i.into());
    }

[gkoz]

Supporting gtk update: https://github.com/gkoz/gtk-rs-gtk/commit/4734f1e4e0f5d9c67039fad4bd877365d9443772

[GuillaumeGomez]

Missing re-exported value I guess.

[gkoz]

I think it's done now. Can't easily run cargo test in travis because of older glib and g_type_init issue but added to appveyor.

[gkoz]

All right. It's the pthread linking errors again. I guess I'll remove the CI changes from this PR.

[gkoz]

Writing the docs made me realize that borrowing the value to get TypedValue might not be the best idea. Instead Value could be converted to TypedValue, which would deref to Value.

[gkoz]

Now TypedValue contains Value and derefs to it.