Add failing tstamp stats test

This test demonstrates how putting on a key more than once in the same
file does not update the oldest/newest timestamp stats for that file.
When re-putting value, update tstamp stats

The old code was passing zero as the timestamp in the case of putting a
value for the same key in the same file. Fixed to use the entry's
timestamp.
Fix EUnit time fudge bug
Merge pull request #152 from basho/bugfix/tstamp-stats

Latest timestamp for entry not recorded correctly