rename main.js to emberui.js

Broccoli has some issues in requiring libraries from bower
https://github.com/broccolijs/broccoli/pull/91
there are attempts to fix this behaviour
https://github.com/joliss/broccoli-bower/blob/master/CHANGELOG.md#020

it could still be good to rename the distribution file to emberui.js
the `global` this and `named-amd` won't be affected (due to how
`broccoli-dist-es6-module` is implemented)

we'll need to test if this solves #21