Fix #235: Custom Map Layers plugins share "clear" event.

We need to bind the "Clear All" click handler to the plugin that it
belongs to instead of binding the click handler to every "Clear All"
link on the page.
Fix #230: Export retry results in too many requests.

The return statement was missing from the terminating condition.

I also added a debounce on the "Trying again X more times..."
message so that it doesn't flash by too quickly before you are able to
read it.
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.
Merge pull request #245 from kdeloach/topic/fix-map-layers-clear-all

Fix #235: Custom Map Layers plugins share "clear" event.
Merge pull request #246 from kdeloach/topic/fix-export-requests

Fix #230: Export retry results in too many requests.
Merge pull request #247 from kdeloach/topic/objectid

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