Add license headers

GPL licenses suggest adding a license notice at the top of each source file, like this:

```

Copyright (C)

This file is part of Foobar.

Foobar is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 2 of the License, or
(at your option) any later version.

Foobar is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with Foobar. If not, see .
```

This is where it is explicitly allowed to use later versions of GPL (the main LICENSE text doesn't specify it). So it's a good idea to add these headers to each file to make sure it's really GPLv2+ and not GPLv2 only.

Relates to #4 as the header can be added along the Roxygen comments.