Allow setting of SSH banner timeouts on Clients

This pull request contains a proposed interface for setting the SSH banner timeouts on Paramiko `Client` objects.

I've been working on using Paramiko as part of an integration testing tool for [Project Calico](http://www.projectcalico.org/). As part of this tool we frequently reset VMs back to snapshots, which can lead to quite long timeouts where the TCP connection is accepted but the SSH daemon is not yet ready to present the banner. These timeouts are frequently more than 15 seconds.

For this reason, it would be useful to be able to set the banner timeout on the underlying `Transport`.

#### Notes

1. This pull request doesn't contain any tests. I wasn't overly concerned, given that I'm just setting a pre-existing value, but if you'd like a test for this I can write one.
2. This is one of a number of possible APIs for setting this value, I'm open to others if you don't like the style.