Add animation duration support

[webevasion]

Add animationDuration to default config variables and replace the animation duration by the variable.
It can be used in the init function.

$(document).alton({
	fullSlideContainer: 'full', // Tell Alton the full height container
	singleSlideClass: 'slide', // Tell Alton the full height slide class
	useSlideNumbers: true, // Set to false if you don't want to use pagination
	slideNumbersBorderColor: '#fff', // Set the outside color of the pagination items (also used for active)
	slideNumbersColor: 'transparent', // Set the inner color of the pagination items (not active)
	bodyContainer: 'pageWrapper', // Tell Alton the body class,
	animationDuration: 375, //Animation duration (in ms).
});