Directive param for centeredSlides

[alvarosabu]

Added param fo directive to setup if the active slider cand be in the center

[kbeaugrand]

I think I will be better to use real swiper api properties.

I should be preferable to use centeredSlides attribute name rather than centered.

In my opinion, We could use all directive attributes to extends our params like :

link: function(scope, element, attrs) {
...
$scope.params = angular.extend({}, $attrs);
...

In this way we could use $scope.params in controller function which will contains all swiper well-named arguments.