README: Clarify the output of some print() calls in the example
Implement limits for the number of times events use an action

This patch implements two methods for controlling how many times an
event can invoke an action.  Luvent enforces this limit for each
individual event even if multiple events use the same action,
demonstrated by the new unit tests.

The patch also introduces a public method for determining if an action
is enabled or disabled.
README: Remove the coroutine example

The example uses a coroutine to remove an action after its first
invocation but it is now better to use the setActionTriggerLimit()
method for that purpose.
README: Add a link to the official documentation for metamethods