Allow pyping to bind to an interface

This patch will allow you to specify an interface on which the socket will connect.
I needed this because I needed to connect over a vpn connection which uses a different gw/route, perhaps it can be of use to someone else as well :)

```python
import pyping
r = pyping.ping('google.com')
r = pyping.ping('google.com', bind='10.125.1.x')
```