refactor(tooltip): lazily evaluate placement, delay

There's no need for an observer here because these values are only used
on initialization of the tooltip.
refactor(tooltip): remove observer for triggers

This change fixes the usage of the tooltip directive with AngularJS
1.3.1 when no trigger attribute is used.

BREAKING CHANGE: tooltip/popover trigger is no longer a watched
attribute.
This affects both popovers and tooltips. The triggers are now set up
once and can no longer be changed after initialization.