Allow ember-admin store adapter's namespace override

Closes #25

You can now re-open the `service:admin` object and change the value of
`namespace`. If it is set to `Ember.isEmpty(namepace) === true` it will
set to `undefined` in the created adapter.

Also adds some unit tests for the `store:admin`.
Merge pull request #35 from dockyard/bc-override-namespace

Allow ember-admin store adapter's namespace override