Update slider and gallery to work with picture element.

I feel like these two methods are a little hard to understand when using the api. Calling horizontal on something, I don't know if it will return it or update it. Maybe `replaceHorizontal` would be better, or even two get and update methods which you would then have to check for type inside. eg.

```
ResrcIt.update = function(type, value) {
  if (type == "horizontal") {
    etc.
  }
}
```

Not very terse but more understandable from the outside. You could even have a bunch of private functions which do this I guess. Something like:

```
ResrcIt.update(type, value) {
  this[type].call(value);
};

// private

ResrcIt.horizontal = function....
```

Just thinking out loud...