Switch to internal mimeParser/mimeSerializer.

[deiu]

We are currently using libmagic to set the correct CTypes. Unfortunately it does not handle RDF types, so we currently need to work around it [1]. This is especially bad in case of really big files.

To fix it, we should just remove the dependency on libmagic and use the stuff we already have in mime.go together with a the check for "@prefix" we do in [1](in case a file extension wasn't detected).

[1] https://github.com/linkeddata/gold/blob/master/server.go#L516