Added labelledby, describedby and required aria properties to the trigger

[ghost]

See cibernox/ember-power-select#254

[cibernox]

Question: {{basic-dropdown ariaLabelledBy="foobar"}} or {{basic-dropdown aria-labelledBy="foobar"}}?

The second seems closer to the html spec.

[ghost]

I think ariaLabelledBy, but I don't really have a good reason or anything for it, it just feels more natural.

[cibernox]

I, in general, prefer to name things in my components as close as possible to the equivalent html spec, however in this case i've seen that ember has out of the box support for aria-role via the ariaRole property, so let's be consistent with ember here.

[cibernox]

Published in 0.8.3