Fixed Makefile.PL to correctly quote hash key 'File::Spec'


Modified method 'invoke' to not check for availability of method.
Instead, just make attempt to call method and let client handle exceptions.
Also, modified API to better handling invocation of methods.  Now just past
as first parameter method name and list of arguments which will be handed
of to invoked method.

NOTE : Only class methods should really be called this way as there is no way
to call the method based on an object.  Invoke is made available as a convenience
method and is not really necessary as once the class is loaded it should be
trivial to just handle calling methods of this class directly.