Tip component (and popover component)

The tip component is a simple container for an inline label and a bit of help text to go along with it in the form of a tooltip.

The popover component is a general purpose pane that is associated with an anchor element. It is the tooltip for the tip component, but it can do more than it. Notably, it encapsulates all the top left fiddling that needs to be done to keep the pane on the screen and the arrow always pointing at the anchor.
The middle guard here is redundant, `Ember.get` will return `undefined` for a non-existent property lookup.