fix for visible/waitUntilVisible when selector matches multiple elements

[Issue #431](https://github.com/n1k0/casperjs/issues/431)

The proposed fix makes it so that, when the selector passed to visible() matches multiple elements, it checks that one of them is visible, as the documentation suggests.

There is no change when passing a selector matching only 1 element.

New test cases have been added and all are passing.