Include cassis

Or document that cassis needs to be loaded to use this library.

As far as I can tell, this line won't work unless cassis is loaded by this library or by whatever app loads this library.

https://github.com/indieweb/link-rel-parser-php/blob/master/src/IndieWeb/link_rel_parser.php#L43

Start at src/IndieWeb/link_rel_parser.php line 43 and inspect how the cassis dependency is used. Confirm whether the library loads cassis itself or requires callers to do so; done means either making the dependency available or documenting the loading requirement clearly.

Written by the indexing model from the issue text.