Live Remote Field Mapping fixes

* Fixed (API Version): Live Remote Field Mapping improvements
	- Fixed endless spinning
	- Empty options are better managed; shows "No Picklist Fields" message
	- Now shows an error message if the field cannot be used for Remote
Field Mapping
	- Static PHP warning fixed when `WP_DEBUG` enabled
Merge pull request #63 from katzwebservices/version-2.6.4

Live Remote Field Mapping fixes