Correct manip generics
Add utils imports
Add data.table deps and require in right places
Remove hack for old version of data.table
Fixes for R CMD check.
* document and export group_by
* add ulevels from plyr
* fix typo in group_by.source_dt
More docs
Remove source name generic and methods
Remove data source names
Improving documentation some more
Move manip docs for grouped dt to own file
More consistent file names