Cast object ids inside of $not queries
Cast object ids inside of $or and $and queries
Cast oids inside of $nor queries
Default query in Collection#find to {}
Default query in Collection#count to {}
Default search in Collection#findOne to {}
Add Collection#save

Note: this takes some ideas from mongolian, in that the second argument returned from
save (1 on success) is not very useful. Instead, this implementation of save returns
the updated or inserted document.
Add tests for Collection#save
Merge branch 'cast-oids-inside-logical-queries'
Merge branch 'default-find-query-to-empty-object'