Use as an HTTP/UDP gateway to Pinba

Pushing data from browsers to Pinba is not possible due to the use of UDP. There exists a project that enables the use of HTTP as a gateway: https://github.com/rs/pinba_http. This pull request adds the same kind of functionality to the nginx module.

A typical use is:

```
  location ~^/track/(.*)$ {
    pinba_enable on;
    pinba_request_timer t;
    pinba_max_timer 10;
    set $pinba_request_uri $1;
    set $pinba_tag_as 3215;
    set $pinba_tag_country FR;
    set $pinba_tag_browser $http_user_agent;
    set $pinba_tag_urgency $arg_urgency;
    return 204;
  }
```

Sending the request `/track/my.event?t=4571&urgency=low` will be translated to a Pinba request with `script_name` equal to `my.event`, `request_time` equal to 4.571 seconds and a timer of the same value with 4 tags: `as`, `country`, `browser` and `urgency`. The latest one is provided by the user. As a safety value, timers greater than 10s are discarded.

The pull request has 3 commits. The two late ones can be used on their own with server-side timing.