Add SpeakerAssociation model

Addition based on patch speaker-association.diff provided by juyrjola
Add option "--dry-run" to eduskunta importer

Option will (pretty)print MP details, but does not commit the changes to the database
Include speakers committee in MP membership parsing
Add SpeakerAssociation saving
Add new speaker association migration 0019
Merge remote-tracking branch 'joona/issue-73'
Fix speaker importing

Closes #73.