Connect timeout

Per discussion with @sigmavirus24 in IRC, this PR kills the Timeout class and adds support for connect timeouts via a simple tuple interface: 

```python
r = requests.get('https://api.twilio.com', timeout=(3.05, 27))
```

Sometimes you try to connect to a dead host (this happens to us all the time, because of AWS) and you would like to fail fairly quickly in this case; the request has no chance of succeeding. However, once the connection succeeds, you'd like to give the server a fairly long time to process the request and return a response. 

The attached tests and documentation have more information. Hope this is okay!