Add support of asynchronous mode.

Hi,

For performance issue, I added an asynchronous mode.
In this mode, I use a socket, so the server does not wait ack.

Because it may be useful to other people, I pull this.

Usage : 
Add the "async" entry in the config array : 
```php
$errBit = Errbit::instance()->configure(array(
    'api_key' => 'YOUR API KEY',
    'host' => 'YOUR ERRBIT HOST, OR api.airbrake.io FOR AIRBRAKE',
    'async' => true, // optional
));
```

Bye