address issue #112

* wrap column names in quotes, even on LHS of `:=` (per https://stackoverflow.com/questions/16172216/data-table-does-not-play-well-with-checkusage)
* use `utils::globalVariables` as suggested in comments of answer: https://stackoverflow.com/questions/15409806/r-package-compilation-with-dependency-on-data-table/15411032#15411032