Replaced dcast.data.table with dcast in examples and tests.
Closes #893. dcast handles factor levels correctly on drop=FALSE.
Minor: use 'is.data.table()' instead of 'inherits(...)'.
Plug in setDT; takes care of null data.table as well.
Removing 'droplevels' in melt. Never came up as FR.
Skip 0-length inputs in dt_na, to use in 'melt' as well.
Remove 'static' from combineFactorLevels, to use in 'melt'.
Added which_ = mem efficient version of which(!x).
Update test number.
bool = TRUE is default for which_
Added simple tests for which_
Added 'which_' in 'unique' & 'setdiff_' & TODO for [.data.table.
Added which_ to na.omit as well.
Closes #982. Allow subset on complex and raw types + tests.
Updated README for #982.
Added the test missed in previous commits.
Added support for complex and raw types to na.omit + tests.