Add registerDateLibrary to allow register every date library & allow ember-truth-helpers v4

[mkszepp]

fix #333

After this changes, there is necessary that the dateUtils will be registered in app.js.

Use case with meta package moment

// app.js
import { registerDateLibrary } from 'ember-power-calendar';
import DateUtils from 'ember-power-calendar-moment';

registerDateLibrary(DateUtils);

For luxon:

// app.js
import { registerDateLibrary } from 'ember-power-calendar';
import DateUtils from 'ember-power-calendar-luxon';

registerDateLibrary(DateUtils);