Add support for reading/browsing an Amazon S3 bucket containing dumps

@alisnic: Here's the support I mention in my last PR for turning an Amazon S3 bucket listing into a browsable page of dumps.

Notable changes:
* Altered the command line syntax -- suggestion is -f for a local dump file, -u for URIs to a dump file and -b for an S3 bucket listing
* Added a .yaml template and corresponding parsing code to deal with the bucket listing
* The "/" route now redirects to either "/overview" or "/listing" depending on which flag was used to start the app.
* Altered the README to match the code.