Use OPML instead of a config file as the feed source

Each summit in the provided opml argument will represent a distinct
river to be uploaded to the given bucket. All the descendents (with
type="rss") of each summit make up

- Drop config file entirely. Move everything to command line
  arguments.

- Clean up stale imports

- Remove some command-line arguments in an effort to clean things up

- Drop template uploading code (for now). Need to find a better way
  for this.

- s3_save: Create a generic S3 uploader function.

- load_opml: Create a generic local/remote OPML parser.
river.py: handle 'include' nodes
river.py: skip commented nodes
river.py: add forget_river function
river.py: fix missing bucket error message
river.py: add --river argument

Useful if you only want to update a single river
river.py: remove unused hashlib import
Remove everything under opml/
Delete sample.cfg
river.py: refactor and split complex stuff into smaller functions
river.py: add ability to clear previously seen rivers