WIP: Restore backup feature

This is work in progress, but I figured I better share early to get comments and ideas.

Basically what this does:
1. Download requested snapshot from S3 (for specified list of hosts, keyspace, tables or all of them). 
2. Invoke `sstableloader` to load the downloaded SSTables into the requested Cassandra hosts.

It downloads files with Boto rather than s3funnel, because I needed to rewrite their name in case we download from multiple hosts.

This can be used to use a backup on your local dev machine, or to copy from production to staging.

Another restore option that should be added is the ability to restore the SSTables into the data directory of the current Cassandra install. This can be helpful when adding a new host to the cluster, that replaces an old one.