Issue #242: Feature Compare plugin expects "OBJECTID".

The following changes were made to the feature compare plugin:

- New layer option in compare_config.json called 'objectIdField' which
  is the name of the OBJECTID property on layer features.
- If no 'objectIdField' is configured, we look for an 'esriFieldTypeOID'
  field in the layer info and use that.
- If all else fails, we revert to the default behavior which just
  assumes that an OBJECTID property exists on the layer feature.