[#25] allow passing storage settings to adapter

- New subkey, storage_settings; when present, passed as second argument
  to adapter in order to further configure it (e.g., set alternate table
  names).
[#25] Implement storage_settings for Mongo adapter

- Follows same pattern as for PDO.
Merge pull request #25 from weierophinney/feature/25

Unable to set custom names for tables