Optimisations + code cleanup for dcast.data.table

+ Moved 'subset' and 'aggregation' from C- to R-side.
+ Also moved ordering/keying data.table at R-level as well.
+ Added an additional test for 'subset' argument.
+ Much needed reduced clutter at C-level, ex: no "cast groups".
+ 2x speed-up due to a) aggregation before ordering whenever possible, b) using internal reordering which is much more efficient - avoids a lot of (unnecessary) mallocs.
+ Updated '?dcast.data.table' with new timings.

+ Minor: Fixed a broken link - "?forder". It's not yet exported (and there's no ?forder yet).

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1225 edb9625f-4e0d-4859-8d74-9fd3b1da38cb