Added possibility to define start and end dates for the export

Usage: copiedbook facebook-page-id access-token output-file [start-date] [end-date]

Examples:
 1) copiedbook 270335753072803 AAACE...3QZA my_file.json
 2) copiedbook 270335753072803 AAACE...3QZA my_file.json "2014-01-01 0:00:00" "2014-06-06 23:59:59"
This is nice, the example on line 28 was terribly awful :)