Box API and datetimes

[ijlyttle]

We have an internal helper-function box_datetime() to parse the the Box datetime format (variation on ISO-8601) into an R POSIXct.

We use this helper manually when parsing responses.

Looking at a bunch of sample API responses at Box, they seem to have an implied contract on names: if it ends with _at it's a datetime - if not, it's not.

It may be interesting to see if we could take advantage of this.