Fix timediff bug & add function to let user pass interval parameter

[keyboardfann]

About elasticsearch2elastic.py will stop while query es too long #7 , please review the code if you accept.

[keyboardfann]

Usage:

## Help ##
[root@xxx eshealthcollector-prod]# python elasticsearch2elastic.py  --help
usage: elasticsearch2elastic.py [-h] [--interval INTERVAL]

Process some integers.

optional arguments:
  -h, --help           show this help message and exit
  --interval INTERVAL  The interval send to ES, unit is second

## Without interval parameter , will use default 60s ##
[root@xxx eshealthcollector-prod]# python elasticsearch2elastic.py 
Interval: 60 s
Total Elapsed Time: 0.864542007446
Total Elapsed Time: 0.867144107819
Total Elapsed Time: 0.743765830994

## With 90s interval parameter ##
[root@xxx eshealthcollector-prod]# python elasticsearch2elastic.py  --interval 90
Interval: 90.0 s
Total Elapsed Time: 0.719194173813
Total Elapsed Time: 0.732607841492