basic support for bootlint 0.5.0

cc: @XhmikosR @cvrebert

I didn't make many changes this evening outside of using the new API and changing `relaxerror` to use the new error codes instead of the messages, but if you guys have suggestions for other features I'd be glad to hear them.

Also move changed the release order on the README and removed some more noise from the output.
We're currently calling them "problem IDs" rather than error codes.