Address documentation issues.
Decision on behavior for factors is yet to be decided. This PR will change behavior so that empty factors are not dropped (by expand, crossing, complete), but "completed." nesting will continue to drop empty factors per the original behavior.
This required a parameter on the "drop_empty" function to alter behavior with factors. factor=FALSE is the legacy behavior.
Added tests with more clarity on different argument types. Also added explicit calls to the dplyr package in some of the relevant tests.