Add element center to positioned container

[wandertosee]

Modal is centered on clicked element.

[lukemelia]

Thanks for putting this together @wandertosee. In order for us to merge this PR, we would need an update to the README docs and the dummy app.

[wandertosee]

I removed the {{view}} helper from the dummy app views but left the README.md and packages.json references to ember-legacy-views (https://github.com/emberjs/ember-legacy-views) for those using <= Ember 2.4.

FYI - "Extended support for Ember.View and friends until Ember 2.4"

Let me know if I can do anything else.

[lukemelia]

Thanks for your work on this @wandertosee!