Rename config/keter.yaml to config/keter.yml for consistency.

These changes are reflected in yesod-bin and keter which check
for keter.yml and then keter.yaml.

Complimentary fix for snoyberg/keter#41.
Merge pull request #28 from creichert/postgres

Rename config/keter.yaml to config/keter.yml for consistency.
Merge branch 'postgres' into mysql