add new module: nginx domain

nginx domain module
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This module works with nginx reqstat module, which will be merged into tengine (https://github.com/alibaba/tengine/pull/260).

It can collect information: connections per second, qps, 2XX-qps, 3XX-qps, 4XX-qps, 5XX-qps, other-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, domain name comes from nginx reqstat module)

nginx reqstat module raw data:

    $ curl localhost/nginx_domain_status
    8892,158,1049353,1,1,1,0,0,0,0,55
    8891,158,1049353,1,1,1,0,0,0,0,42

raw data format:
domain_name, in_bytes, out_bytes, connections, requests, 2XX-requests, 3XX, 4XX, 5XX, other, response_time

some trivial patches
---
fix compiling warning (item_value is unused)
fix wrong alignment of mod_nginx help info