makeElementsVisible property

KIF automatically scrolls elements into view. Sometimes this causes major issues because of numerous `UIScrollView` bugs. This pull request adds a `makeElementsVisible` property to `KIFUITestActor`. The default is `YES`. When set to `NO`, the automatic scrolling is disabled.

I considered creating a bunch of new `waitForElement` methods with a `makeVisible` argument, but this would really balloon `KIFUITestActor`. Since most users will not need to disable scrolling, using an unobtrusive property seemed like a better approach.

Let me know if this pull request is acceptable to everyone.