Complete broken functionality and add option to disable ability to disable notifications

The contained commits:

* Add system plugin setting to enable debug mode. Remove Log4MT lines from `NotifyWho.pm`, replace with native Activity Log debug dumping.
* Restore notification in MT5+ upon creating a draft. In MT4, the cms_post_save.entry callback returned nothing for the original object, whereas MT5+ returns a simple object.
* Notify upon setting draft entry to scheduled, or upon creating a scheduled entry if the “on create” setting is disabled.
* Notify upon publishing through Edit Entry screen an entry previously created as a draft. The hidden field associated with the notification toggle must be present to trigger the notification, but it was being omitted from Edit Entry if the entry was not new.
* Fix the “on create” and “on publish” settings to work sanely and be clearer. Previously both boxes had to be checked for any notifications to be sent.
* Notifications are now sent when draft entries are published via the Manage Entries screen or published or scheduled via Batch Edit entries function. The separate callback cms_post_bulk_save.entries is used for those functions, hence they were not picked up with cms_post_save.entry. Also, code was requiring a hidden form field on the Edit Entries screen, which does not apply to the bulk update methods.
* Add setting to prevent users from being able to disable notifications on the Edit Entry screen. This allows the plugin to function better in a work flow that requires an audit trail.
* Bump to 2.1.0, minor readme adjustments.