Merge pull request #2 from chrisavl/master

Fix infinite recursion in map and foldl
Helper for shifting a date on day.
Helper for yesterday.
More shift helpers.
Format as ISO8601.
ISO8601 basic.
Diff.
Enumerate the days in a range.
Allow map and foldl to handle hours
Merge pull request #3 from tgrk/master

Allow map and foldl to handle hours
Badarg on incorrect input.
Support hours interval in range