add nginx domain module

This module works with nginx reqstat module, which is merged into
tengine (https://github.com/alibaba/tengine).
It can collect information: cps, qps, 2XX-qps, rt per domain.

An example is as following:

$ tsar --nginx_domain -li1
Time              --------------8891-------------- --------------8892--------------
Time                 cps     qps     2XX      rt      cps     qps     2XX      rt
22/10/13-15:44:29 797.00  797.00  797.00    2.06    20.00    2.0K    2.0K    0.00
22/10/13-15:44:30 607.00  607.00  607.00    2.74    20.00    1.9K    1.9K    0.00

(8891 and 8892 mean listening port of subdomain)
mod_tcprt: delete unused variable "item_value"
make mod_nginx help info (tsar --help) aligned
mod_nginx_domain: add status info: 3XX, 4XX, 5XX
Merge pull request #25 from chobits/master

add new module: nginx domain