Shapes 3D effects can specify top and bottom bevels like in the
following example:

  <a:sp3d z="488950" extrusionH="63500" contourW="50800">
    <a:bevelT w="139700" h="88900" prst="cross"/>
    <a:bevelB h="88900" prst="relaxedInset"/>
  </a:sp3d>

This patch preserves the a:bevel* tags and their attributes using the
shape grab bag and modifies an existing unit test to add this check.

Change-Id: I4762111e4d2f75ba2fd3721a126aa324a28a853c