Re enable rotation button transition

The opacity transition was broken with #2782

Tested with FF 32 and chrome 36
delay has to be reset (& just for `visibility`): I thought of something like:
```css
.ol-rotate {
  transition: opacity .25s linear, visibility 0s linear;
}
.ol-rotate.ol-hidden {
  transition: opacity .25s linear, visibility 0s linear .25s;
}
```