`ctrl+c` now correctly terminates `fab serve`

Previously `ctrl+c` a `fab serve` wouldn't necessarily terminate the web server. Even if it does, re-using the command `fab serve` might result in the following error:

```
socket.error: [Errno 48] Address already in use
```

This fix manually creates a `TCPServer` with `allow_reuse_address` set to `True`, which solves this issue.

Tested on OS X 10.9.1.