status: add 308 because of RFC7238
Extract status reasons into separate file

This makes rubocop happy about class length. Probably this needs to be reported
to rubocop instead: Hash should be treated as a single line if it's a plain
Hash like this one.

On another hand, it's now possible to require reasons without Status class
definition, e.g. in reel.

/cc @sferik @tarcieri