add docs to show how to apply a function rowwise
Merge branch '124-rowwise-docs' into develop
Add test that `.id = NULL` drops `.id`
Handle `.id = NULL`

Make it work per what the documentation says it does.
Easier fix

See https://github.com/hadley/plyr/pull/207
Merge remote-tracking branch 'dougmitarotonda/master' into develop