Change plugin structure to ensure it can self-deactivate when the feature is merged into Core

[audrasjb]

This pull request moves everything into functions.php and checks if wp_is_plugins_auto_update_enabled() functions exists before running the plugin’s functions.

Fixes #34

[pbiron]

Just pushed one minor change: adding a newline to the end of main plugin file. Not sure if that is required by WPCS, but is generally good practice (IMHO).

Other than that, this works as expected. I tested it by adding function wp_is_plugins_auto_update_enabled() {} to wp-config.php of a test site and verified that functions.php is not loaded and the plugin becomes a no-op.

So, I think this is ready to merge.

[audrasjb]

Thanks @pbiron!