Expand docs to include setting up Grocer.pusher for testing

I wanted to add tests for grocer but found they kept hanging until I set up `Grocer.pusher` differently for `Rails.env.test`. I expect other people have had the same issue so thought it might be worth expanding the docs.
@stevenharman

Good point. Did some experimenting.

The test is:

```
 it "sends push notification for mentions" do
    post = create(:post, words: '@jxdx pick up wine', user: @dan)

    Timeout.timeout(3) {
      notification = @server.notifications.pop # blocking
      expect(notification.alert).to eq 'dan mentioned you'
    }
  end
```

It fails with:

```     
Failure/Error: notification = @server.notifications.pop # blocking
     Timeout::Error:
       execution expired
```

I added the above code to fix that, but it turns out that it is removing the gateway line that fixes it; the certificate definition can be left in without causing any problems. Drilling back further this is due one of our devs defining production and sandbox gateways explicitly. I guess he missed the note:

> gateway: Defaults to different values depending on the RAILS_ENV or RACK_ENV environment variables. If set to production, defaults to gateway.push.apple.com, if set to test, defaults to localhost (see Acceptance Testing later), otherwise defaults to gateway.sandbox.push.apple.com.

as it's up top, not in the Acceptance Testing section. Maybe add a reminder to the Acceptance Testing section or make this more clear in some way.