Upgrade to PHP 7.4, dev-only mocks, etc.

Update changelog.

Standardize on this for Producer 3.

Make non-empty.

Update required PHP version to 5.4.

Corrected PHP version.

PHP 5.4 does not support initializations using expressions containing constants.

Updating README.md to match previous commit.

Doing so separately to experiment with semantic release numbering.

That's my story, and I'm sticking with it.

Add syslog parameters getters/setters.

Provides getter and setter methods for the standard syslog parameters:
* Facility
* Ident
* Options
* Minimum priority level below which messages are not logged

Hotten the links by adding the URLs.