title: Find Google+ avatar for given email address without OAuth
body: <p>In my app I want to show the Google+ profile picture for a user.
  The only function, I found, in the API to get the profile picture needs a userId.
  However, I only have their email adress and not their Google+ userID.<br>
  Moreover the person, whose image I want to get, should not be forced to log in and authorize my app, as this person is mostly not identical to the user of the app.</p>
  
  <p>So I think I need to get their userId by email. I read through the Google+ API documentation but can't find a way to do this, but I can't believe that this is not possible.</p>
  
  <p>So my question is:<br>
  How can I get the Google+ userID with only an email address?<br>
  Is there maybe an other Google API to get a profile picture?</p>