replace with progressview upon login button click as suggested by tkriz

as suggested by @tkriz let's add a UIActivityIndicatorView in-place of the login-button when clicked

done for [AGPUSH-899](https://issues.jboss.org/browse/AGPUSH-899)

@sebastienblanc @corinnekrych @matzew mind to give it a spin?
sure @TadeasKriz but.. couldn't find as much as I tried on how to reset the button to the one I used in the IB already [here|https://github.com/aerogear/aerogear-push-quickstarts/blob/master/client/contacts-mobile-ios-client/Contacts/Controllers/AGLoginViewController.m#L28]

if u have some suggestion on this would be glad to hear and update!