Fix authentication to use Amazon CSRF tokens

- Dropped cp_token in lieu of csrf_rnd, csrf_token, and csrf_ts
- Use safer getpass instead of using password parameter
- Use AUTH_URL to get Amazon's OpenID page for the signin form, rather than the old Cloudplayer signin form.
Moved getpass out to command-line, log auth failures.
Fixed duplicated CSRFTokenConfig reference
add retries and timeouts to requests
add download-all command, including a 'greater than' operator for handling large libraries
refactor dry_run a little, add a 'skip existing files' case too.
Merge pull request #4 from tedder/authentication_fix

Authentication fix + download-all