js: getDefaultKey and verify fix

• DemoExtension/maindemo.js - added a Demo for retrieving the default signing key

• src/Errors.js - add a new Error if no default key can be determined

• src/Key.js added documentation and a TODO marker for hasSecret.

• src/Keyring.js implemented getDefaultKey

• src/permittedOperations.js: Added missing entry for verify, added config_opt