Remove the private function addActionToEvent()

The function is no longer necessary now that we have addAction() as
the sole public method for adding actions to events.  Previously we
needed this function to help with addActionWithInterval() but now
that the method is no longer part of the API we simply the code base
by getting rid of this internal function.
Add a unit test for disabling and re-enabling actions
Only invoke enabled actions

This patch adds an ‘enabled’ property to every action.  An event will
only invoke enabled actions.  All actions start out enabled and this
patch provides no way to toggle their status via the public API.
Omit unit tests tagged with ‘#delay’ by default

This patch changes the default Busted configuration so that it will
skip the most time-consuming tests.  But the patch also provides a
second configuration so that we can use

    $ busted --run=all

to execute every test in the test suite.
Implement enableAction() and disableAction()
Add a unit test to ensure Luvent does not remove disabled actions

We want to leave the disabled actions in the event, otherwise we would
not be able to re-enable them later.
README: Add disableAction() and enableAction() to the API