Use new bytestring syntax
Merge pull request #4 from andor44/bytestrings

Use new bytestring syntax
Cargo support
Updated readme for cargo
Updated for latest master
Key::from_str now takes &str instead of &'static str (fixes #5)
Allow `Key` to be constructed manually

Tuple structs are now allowed to have private fields which means
you can not manually construct a Key with the old syntax.