added 2 new methods

As discussed in issue #15, added two new methods that can be used as
drop-in replacement to timeAgo: dateTimeAgo and dateTimeUntilNow. See
header file for more details about those methods.

Also refactored some code in timeAgo to simplify it.
Merge pull request #19 from chrisben/master

Issue #15 : added 2 new methods