Add the ability to specify the end timestamp in human-form

Summary: Before, we would only accept the UNIX epoch time for the end time stamp to grep the logs for.  This works well for machines, who think about time in the number of seconds since 1970, but not for modern humans, who usually think about time in the context of a gregorian calendar.  So, I made this parameter try to detect which format you are specifying and convert accordingly.

Test Plan:
Ran it a few times on ka-analytics machine:

```
analytics@ip-10-0-0-108:~$ greplog -n -t "2013-07-21 23:00:00" -d 1400 "student_use_data_report"
Searching ['/home/analytics/kalogs/2013/07/20/backends-23:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-00:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-01:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-02:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-03:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-04:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-05:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-06:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-07:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-08:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-09:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-10:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-11:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-12:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-13:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-14:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-15:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-16:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-17:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-18:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-19:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-20:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-21:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/21/backends-22:00:00Z.log.gz']
for all requests in the 1400 minutes before 21/Jul/2013:23:00:00
where it contains 'student_use_data_report'
```

```
analytics@ip-10-0-0-108:~$ greplog -n -t "2013-07-21" -d 1400 "student_use_data_report"
Searching ['/home/analytics/kalogs/2013/07/20/backends-00:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-01:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-02:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-03:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-04:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-05:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-06:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-07:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-08:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-09:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-10:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-11:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-12:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-13:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-14:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-15:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-16:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-17:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-18:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-19:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-20:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-21:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-22:00:00Z.log.gz', '/home/analytics/kalogs/2013/07/20/backends-23:00:00Z.log.gz']
for all requests in the 1400 minutes before 21/Jul/2013:00:00:00
where it contains 'student_use_data_report'
```

```
analytics@ip-10-0-0-108:~$ greplog -n -t 1381790970 -d 1400 "student_use_data_report"
Searching ['/home/analytics/kalogs/2013/10/13/backends-23:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-00:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-01:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-02:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-03:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-04:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-05:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-06:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-07:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-08:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-09:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-10:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-11:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-12:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-13:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-14:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-15:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-16:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-17:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-18:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-19:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-20:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-21:00:00Z.log.gz', '/home/analytics/kalogs/2013/10/14/backends-22:00:00Z.log.gz']
for all requests in the 1400 minutes before 14/Oct/2013:22:49:30
where it contains 'student_use_data_report'
```

Reviewers: james

Reviewed By: james

CC: marcia

Differential Revision: http://phabricator.khanacademy.org/D4647