Use defaultUser.png instead of defaultUser.gif

Indeed, defaultUser.gif is in a deprecated skins folder (Products/CMFPlone/
skins/plone_deprecated/defaultUser.gif) and not defaultUser.png
(Products/CMFPlone/skins/plone_images/defaultUser.png).
Test is also updated.
Merge pull request #37 from plone/bsuttor-use-defaultUser-png

Use defaultUser.png instead of defaultUser.gif