Make ember g adapter extend from application adapter if present (closes [#1764]).
Add --base-class options to ember g adapter.

Allows to specify a --base-class or use app/adapters/application.js if
present.

Expects base-class to be relative to /adapters.

- `ember g adapter --base-class=application`
- `ember g adapter --base-class=bar`
- `ember g adapter --base-class=foo/bar`
Merge pull request #1831 from abuiles/adapter-generator-enhancement

Make ember g adapter extend from application adapter if present (closes ...