Can't test hosts that require client certificate authentication.

According to heartbleed.com:

> Does TLS client certificate authentication mitigate this?
>
> No, heartbeat request can be sent and is replied to during the handshake phase of the protocol. This occurs prior to client certificate authentication.

However running this tool against a host requiring client certificate authentication results in:

```
rickette@rickette ~/W/Heartbleed> ./Heartbleed somehost:443
2014/04/08 12:06:18 somehost:443 - ERROR: remote error: handshake failure
```

It works fine if this host doesn't require client authentication