Accepted changes from #11

Wanted to make a separate issue from #11 in order to track completion of accepted changes.

[x] owner -> group, makes the purpose more clear
[x] name -> id, also makes the purpose more clear
[x] add optional name, human-readable title, i.e. "My Package"
[x] add authors, but optional. format along the lines of package.json's authors
[x] add credits, optional. perhaps call it contributors instead
[x] make dependency versions optional

craftson / spec