Allow enable() and disable() methods to be used when auto_events is TRUE

[ashbythorpe]

Fixes #144.

Works by keeping track of which domains have been manually enabled, and does not call enable() in inc_event_callback_count() or disable() in dec_event_callback_count() until the corresponding disable() method is called.

If the event callback number is more than 0, and disable() is called, automatic event handling is disabled until the callback count reaches 0 (not by design, just due to how the callback count functions were written in the first place).