add alarm-segfault and allow to push segfaults to airbrake compatible er...

...ror trackers (tested with errbit)

This is updated patch that adds:

* ```--alarm-segfault``` option for raising alarm on segfault in uWSGI
* format airbrake notification xml and post it to airbrake compatible servers using alarm_curl plugin, usage:

```
plugins = alarm_curl
alarm = errbit curl:http://airbrake.server/notifier_api/v2/notices;airbrake=<AIRBRAKE API KEY>;subject=uWSGI Segfault
alarm-segfault = errbit
```

and cause uWSGI to segfault, traceback will be recorded in airbrake server.

We could even setup public errbit server and start collection segfaults from uWSGI users, but I fear that soon they would contain:
```

/uwsgi:0→ buy_cheap_viagra()
/uwsgi:0→ enlarge_anythig()
/uwsgi:0→ ()
```

This fixes #145 

Airbrake API docs are [here](http://help.airbrake.io/kb/api-2/notifier-api-version-23)