Support units in floor_date() etc

[hadley]

i.e.

floor_date(now(), "2 weeks")

[yeedle]

was looking for similar functionality for rounding dates, e.g. round_date(x, "5 minutes")

[vspinu]

I have implemented this and added quite some tests, but would really appreciate if you guys could play with it a bit and run your own checks on corner cases.

Multiple of all units excepts weeks are supported (updating on weeks is not supported by POSIXlt) . Rounding on units that don't have a meaningful 0 (days and months) happens incrementally from 1:

> floor_date(ymd("2016-08-02"), "2 days")
[1] "2016-08-01"
> floor_date(ymd("2016-08-03"), "2 days")
[1] "2016-08-03"

This is consistent with how currently bimonthly, quarter and halfyear behave.