make sense of min_rank(desc(.))
handle desc in min_rank. #149
handle row_number(desc(.)). closes #149
initial lead implementation. #133
lead impl for Date and Time.
TypedPromoter was losing class
same for ConstantTypedGatherer
Lag initial impl (does not handle default or order_by yet)
handling date and time classes. #167
commenting lag and lead so that I can merge into master
Merge branch 'v0.2'
Fixes Date which can be both INTSXP and REALSXP.
Also includes lead and lag implementation, but not enabled yet because it does not handle all arguments yet.