Add setOwner support

[jamesarosen]

See #6 (review)

[jamesarosen]

The failing test is because ember-test-helpers calls setOwner with container.owner, but this polyfill doesn't tell the container that its owner is the FakeOwner.

Some ideas:

• change the getOwner and setOwner polyfills to set object.container.owner. It might be good to also change getOwner to check for object.container.owner before creating a new FakeOwner
• change the setOwner to no-op when owner is null, though this is different from the real implementation, which allows setting to null

We may need to change ember-test-helpers to call setOwner(this.context, getOwner(this)) instead of pulling the owner off the container directly.

Or we might be able to have this polyfill change the way Containers are created to give themselves an owner.

[rwjblue]

@mmun - OK, CI is happy, the tests look good, any objections to landing?