Set external id when creating new asset.

External id, which is effectively the national bus stop id, is generated from national_bus_stop_id_seq sequence that starts from 300000.
Migrate national bus stop sequence to database
Merge branch 'master' of github.com:finnishtransportagency/digiroad2 into populate_external_id_when_creating_asset
Merge pull request #150 from finnishtransportagency/populate_external_id_when_creating_asset

Populate external id when creating asset