Create lib/dates

Several modules were importing datetime format strings from the UDF
module. I wanted to add a helper function that made use of our
"standard" format string but, While not duplicating string literals is
good, entangling unrelated modules is less good.
Add unix timestamp helper

The Stripe APIs use Unix timestamps. This helper wraps up the somewhat
ugly method of creating one in Python.
Merge pull request #1748 from jwalgran/topic/create-lib-stringdate

Create lib/stringdate