Fix the definition of isValidAction()

The document claims that the function returns a boolean but this is
not what the code does.  This patch changes the code so that it
actually implements the behavior described in the documentation.

The patch also makes addAction() raise an error if given an argument
for which isValidAction() is false.  This change maintains the
existing behavior.
Do not raise errors in isValidAction() over metatables without call()

The code attempts to directly access the `__call` property of a
metatable but that property may not exist in the table.  This patch
changes the lookup to a format that will not cause an error in the
event that the metatable has no call() metamethod property.
Implement equality comparisons for events
README: Add installation instructions
README: Describe plans for features in future versions
Rename Luvent.new() to Luvent.newEvent()