Tasks to lint or convert YAML/JSON samples

Replaces https://github.com/reverb/swagger-spec/pull/79 since the schema validation is no longer needed (it's handled by both mocha and sbt).

I threw together a few Gulp tasks that:
- Go through all the YAML and JSON files and "lint" them (check for errors, common mistakes, formatting)
- Can convert from YAML to JSON or JSON to YAML
- ~~Validate samples/examples against the Swagger schema.~~ (This is now handled by mocha & sbt)