Don't show activity indicator when imageURL is nil

When nil is set as imageURL the activity indicator is shown and never
hidden. This pull request just doesn't show it in the first place.
Merge pull request #15 from Shukuyen/master

Don't show activity indicator when imageURL is nil
Merge pull request #23 from jweyrich/master

Fix warnings caused by -Wsign-conversion and -Wshadow-ivar.