PERF: add vbenchs for groupby functions with different group sizes

[jreback]

http://stackoverflow.com/questions/22845856/performance-issues-with-groupbys-last-in-pandas/22846215#22846215

want to run all built-in functions (e.g. first,last,nth,sum,min...etc with
large number of groups, say 10000 to verify perf.