Null Image URL

I ran in to a null pointer exception while using the app today. It looks like the image URL can sometimes be null. 

Stacktrace below: 

```
java.lang.RuntimeException: Could not dispatch event: class cz.destil.wearsquare.event.ImageLoadedEvent to handler [EventHandler public void cz.destil.wearsquare.activity.CheckInListActivity.onImageDownloaded(cz.destil.wearsquare.event.ImageLoadedEvent)]: null
	at com.squareup.otto.Bus.throwRuntimeException(Bus.java:456)
	at com.squareup.otto.Bus.dispatch(Bus.java:386)
	at com.squareup.otto.Bus.dispatchQueuedEvents(Bus.java:367)
	at com.squareup.otto.Bus.post(Bus.java:336)
	at cz.destil.wearsquare.util.MainThreadBus.post(MainThreadBus.java:18)
	at cz.destil.wearsquare.util.MainThreadBus$1.run(MainThreadBus.java:23)
	at android.os.Handler.handleCallback(Handler.java:733)
	at android.os.Handler.dispatchMessage(Handler.java:95)
	at android.os.Looper.loop(Looper.java:136)
	at android.app.ActivityThread.main(ActivityThread.java:5026)
	at java.lang.reflect.Method.invokeNative(Native Method)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:602)
	at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
	at cz.destil.wearsquare.activity.CheckInListActivity.onImageDownloaded(CheckInListActivity.java:84)
	at java.lang.reflect.Method.invokeNative(Native Method)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.squareup.otto.EventHandler.handleEvent(EventHandler.java:89)
	at com.squareup.otto.Bus.dispatch(Bus.java:384)
	... 13 more
```

Nice work on the app btw. I'm a big fan!