Holiday calendar aware date offsets

[changhiskhan]

for business freq (B, BQ, BQS, BM, BMS, BA, BAS)

xref: http://stackoverflow.com/questions/13462802/dateoffset-in-pandas-with-a-holiday-calendar

[rhaskett]

A couple more date offset suggestions:

1. allow "if dt in offset" for more readable code
2. Some support for picking out months in a year. The third wed in Mar, Jun, Sept and Dec would be a common finance offset.

thanks

[jreback]

closed via #3744

[rhaskett]

Thanks a bunch.