Added Pry to the test group to aid in debugging.
Added Pry to the testing suite.
Fixed failing datetime tests.

The failures were specific to xlsx format spreadsheets. When setting the date time to a string format, we need to round to the nearest second.
Fixed edge case involving date time string with seconds.

When supplying a date time string with seconds (11/5/2013 11:45:00), the resulting DateTime object would not round the fractional seconds resulting in a time string of 11/5/2013 11:44:59. 

Time is now properly rounded before returning the DateTime object.
Merge pull request #104 from jrmyward/fix_xlsx_datetimes

Fix xlsx datetimes