Add sync option to BOSH

Fix for metajack/strophejs#16 and strophe/strophejs#47.

I used part of the patch @Gordin proposed in strophe/strophejs#47 in order to bring this feature finally to strophe :).

Use e.g. like:

```javascript
// _connection == Strophe.Connection instance
_connection._options.sync = true;
disconnect();
_connection.flush()
```