feat(tooltips): Add tooltip directive

Closes #552

Hat tip to @glynnis for doing the actual hard work on this one
I know it takes up more space, but I really prefer having all properties on their own line. It can be very inconsistent to go back and forth between the two and makes reading the actual styles much harder.

FYI, [the style guide covers this](https://github.com/rackerlabs/encore-ui/blob/master/guides/css-styleguide.md#format).