Mutabal, potato pancakes and raw avocado

This is a beta version with basic support for transitions and animations.

When the dropdown is rendered, it gets a .ember-basic-dropdown--transitioned-in class after it's first rendered, so it's trivial to add transitions when opened.
The closed, the dropdown gets a ember-basic-dropdown--transitioning-out class, and if there is any CSS animation or transition on the dropdown, the select is not really closed until that animation finishes.

All this behaviour is actually part of ember-basic-dropdown.

This is the kind of feature that might have corner cases depending on the browser, so please, test it and report.