Add a setting to use a default scheme for parsing

[smola]

As discussed in #35, I'll try to add the possibility to use a default scheme via URLParsingSettings. This is useful for parsing URLs introduced by users where a full absolute URL is expected but the user misses the scheme (e.g. example.com, not http://example.com).

[smola]

There is ongoing work in the feature/default-scheme branch: https://github.com/smola/galimatias/tree/feature/default-scheme

Still trying to figure out a sane behaviour.