Fix for #5307 - aggregation with logical columns sometimes gave wrong results

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1103 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
forder now accepts single vectors, and 'by' any subset of columns.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1104 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'
forder now actually works on single vector input, thanks Arun.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1105 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'