Improve help message for -t and add -n for searching backends logs

Test Plan:
1. scp greplog.py ka-analytics:/home/analytics/analytics/tools
2. On analytics machine, pass -h see new help message
3. Execute these commands:

analytics@ip-10-0-0-108:~$ greplog -d 1440 15803433321911ACB63FE
Searching ['/home/analytics/kalogs/2013/08/14/19:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/20:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/21:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/22:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/23:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/00:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/01:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/02:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/03:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/04:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/05:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/06:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/07:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/08:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/09:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/10:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/11:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/12:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/13:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/14:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/15:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/16:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/17:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/18:00:00Z.log.gz']
for all requests in the 1440 minutes before 15/Aug/2013:19:00:00
where it contains '15803433321911ACB63FE'

analytics@ip-10-0-0-108:~$ greplog -n -d 1440 15803433321911ACB63FE
Searching ['/home/analytics/kalogs/2013/08/14/backends-19:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/backends-20:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/backends-21:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/backends-22:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/14/backends-23:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-00:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-01:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-02:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-03:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-04:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-05:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-06:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-07:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-08:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-09:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-10:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-11:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-12:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-13:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-14:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-15:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-16:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-17:00:00Z.log.gz', '/home/analytics/kalogs/2013/08/15/backends-18:00:00Z.log.gz']
for all requests in the 1440 minutes before 15/Aug/2013:19:00:00
where it contains '15803433321911ACB63FE'

Reviewers: james

Differential Revision: http://phabricator.khanacademy.org/D3585