title: Unknown attribute android:elevation
body: <p>I'm trying to follow a tutorial that suppose to show how to add a floating button and in the tutorial it says to add the attribute <code>android:elevation</code> to the buttons xml like this:</p>
  
  <pre><code>&lt;ImageButton
      android:layout_width="30dp"
      android:layout_height="30dp"
      android:id="@+id/editButton"
      android:layout_gravity="center|right"
      android:clickable="false"
      android:background="@drawable/edit_grey"
      android:layout_marginRight="1dp"
      android:elevation="@dimen/elevation_low"/&gt;
  </code></pre>
  
  <p>but it dosen't recognize that attribute...i believe it has something to do with my project target or sdk...can somebody help me?</p>