Refactored couch.Couch methods that query a provider's docs

- Changed these methods so that the one for the "dpla" database
  can use the built-in _all_docs view.
- Removed dpla database view all_provider_docs/by_provider_name
Cleaned up export_database.py to use couch module for views

Added couch.Couch.dpla_view and couch.Couch.all_dpla_docs