Implement getNthBusinessDay()

Summary: I will need it for nagging tool.

Test Plan:
None yet.
Please suggest me how to create a testing database (I need to insert some data in the table). I guess that it is now possible?
There is also probably some bug in arc unit - setEnvConfig() is not called before getEnvConfig() resulting in fatal error.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2376