Automatic index column generation for dcast?

http://stackoverflow.com/a/31194461/559784

``` r
 setDT(df1)[, ind:= 1:.N, .(zip, acs.pop, napps, pperct)]
 dcast(df1, zip+acs.pop + napps+pperct~ind, value.var=c('zgrp', 'perc'))
```

This kind of operation is required quite frequently before `dcast` (AFAICT from Q on SO).
