fastorder forwards now switches to iradix if the first column has range>1e5,  and special cased subgroup size=2

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1081 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Creorder now ignores the beginning and end of ordering vectors where that region doesn't change the row ordering,  such as c(1:500, 502, 501, 503:nrow).   Creorder(DT, 1:nrow) will now return instantly.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1082 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Improved error checks in Creorder

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1083 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added check to Creorder that  start <= o[start:end] <= end, just to be safe and robust.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1084 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'