More checks and add missing license headers

[GuillaumeGomez]

Based on gtk-rs/checker#16.

Fixes #60.

For information: I used this command to replace most license headers:

$ sed -i '/^\/\/ See the COPYRIGHT file at the top-level directory of this distribution.$/d' */src/*.rs && sed -i '/^\/\/ Copyright 20/d' */src/*.rs && sed -i 's;^// Licensed under the MIT license, see the LICENSE file or <https://opensource.org/licenses/MIT>;// Take a look at the license at the top of the repository in the LICENSE file.;' */src/*.rs && sed -i 's;// Licensed under the MIT license, see the LICENSE file or <http://opensource.org/licenses/MIT>;// Take a look at the license at the top of the repository in the LICENSE file.;' */src/*.rs