Fix rng gem

[ronaldtse]

The rng gem is meant to parse RELAX NG and RELAX NG COMPACT (RNC) schemas and provide a DSL for building them.

Currently the specs are failing.

The task is to create a fully working version of the gem that can read and round-trip RNG and RNC schemas. We need to add spec/fixtures files of known examples of RNG and RNC for round-trip testing purposes.