3.x bug fixes and test coverage

• #227 - Move ember-wormhole from devDependencies to dependencies
• #228 - Ensure event listeners are removed onDestroy
• #234 - Check for popperInstance before editing classes
• #245 - Add escapeWithReference to fix positioning edge case
• #256 - Check for isDestroyed and isDestroying before sending onDestroy() action
• #266 - Incorporates relevant 2.x branch changes into 3.x
  • Ember 3.x support
  • Replace SASS/SCSS with plain CSS
  • Remove deprecated use of sendAction
  • Fix tooltip rendering under ember-wormhole 0.5.4
  • Fix CI build for 3.x
• #268 - Fix performance bug by preventing layout thrashing in popper onUpdate hook
• #269 - Introduce major updates to the test suite
  • Support Ember 3 test syntax
  • Use addon-test-support dir (new path for helpers is import { assertTooltipVisible } from 'ember-tooltips/test-support')
  • Remove triggerTooltipTargetEvent() in favor of ember-test-helpers' triggerEvent()
  • Remove afterTooltipRenderChange() in favor of using await with async helpers (see Readme for examples of how to use test helpers)