New features for thunder-ec2

These two commits implement the following additional functionality is the thunder-ec2 script:

**1. The ability to start/stop a cluster**
This is useful for us because we often work on the same cluster over time, installing third party libraries and maintaining other various bits of state on the underlying EBS volumes.  It's also a little faster to start/stop a cluster than it is to create/destroy one.  To implement this, I basically just pulled over the required code from the spark_ec2 script, though I also added some additional ssh options to prevent known_hosts warnings from cropping up as ec2 IP addresses change.

**2. SSH port forwarding**
This is a convenient and secure alternative to opening a port in the ec2 management console and connecting to iPython notebook over SSL. Using ssh tunnels saves a trip to the ec2 security settings page.  The tunnels work great as long as you are willing to remain logged into your cluster over ssh when you are using it.  Also added a setup-notebook-sshtunnel script that configures iPython notebook and thunder for use in this mode.