Bug fixes for #5296 and #5321.

+ Fix for #5296. `[.data.table` now retains the same class as the input data.table.
+ Fix for #5321 - handling POSIXlt columns during data.table creation.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1210 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Fixed 'object not found' error

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1211 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'

Conflicts:
	pkg/NEWS
	pkg/inst/tests/tests.Rraw
More savetl defending.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1212 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'
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.