Use Time.now instead Time.new

* some tests failed with Time.new (?)
Use '+' on time_stamp calculations

* enlarge the MAX_OFFSET a bit for edge cases with randomization
Work around a strange yard bug ?
Use TimeStamp in Fact and Graph (instead of Time)

* still need to refactor the time fiddling from Graph to
  the TimeStamp class
Work-around for a jruby bug

* reduced the far_future from 2500 to 2250 as work around for
  http://jira.codehaus.org/browse/JRUBY-7095 (time overflows > 2250)