Support GVariant format

Fixes #21 (closed).

TODO: I realized very late how much incompatibility is there between D-Bus and GVariant serialization so ended up with a lot of format checks in ser/de code. I'll create an issue for that and clean up the code later.

Issue: Due to conflicts with the blanket generic From implementation, it is not possible to provide direct conversion from Option<T> to Value or even the Maybe type. Let's just hope that people don't use Maybe types a lot inside variants.