Enable custom boto S3 connection configuration

The config file can now have the following S3-specific config values:

  s3_host
  s3_port
  s3_debug
  s3_proxy
  s3_proxy_port
  s3_proxy_user
  s3_proxy_pass

These config values will be forwarded on to the Boto S3 connection constructor
Factor out s3_config settings to boto_configs
Remove the s3_host and s3_port settings

Because they didn't work before this changeset, this is a safe change.
The documentation has been updated to show how to specify the S3 host.
Merge pull request #299 from hufman/feature-private-s3

Enable custom boto S3 connection configuration