setorder sets 'key' to NULL by default now. Not sure if it makes sense for setorder to check and retain key in certain special cases, but that's to revisit later.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1213 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Another corner case in 'setorder'. setorder(NULL) now returns 'x', not 'invisible(x)'.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1214 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added '?setorder' which also includes 'setcolorder'. More changes to '?setkey' will follow.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1215 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
A new separate `?setDT`. Cleaned up `?setkey`.
Fix for #5366 - setkey works properly on data.table containing list columns. Tests added.
Merge branch 'refs/heads/r-forge-git-svn-local'

Conflicts:
	pkg/inst/tests/tests.Rraw
	pkg/man/setkey.Rd
Closes #5408 - order of columns were sometimes reversed with as.data.table.table. Tests added.
Minor corrections to '?setorder'.