Extract RRClass
Add RRClass generation script to git
Beginning renovations.
Add OpCode, RCode, and Class enums.
Implement error::Error for dns::errors::IdentifierError
Move IANA numbers to `dns::number` and add EDNS0.
Remove .DS_Store and add pattern to .gitignore

[skip ci]
Merge pull request #4 from oko/redesign

Structurally this is much better than the original codebase. Will build new features in additional PRs.