Control avatar size when uploaded

Hi guys,

We needed to have a check on the size of avatars uploaded by users. It was as a TODO in the changeavatar.vm
so I follow you my implementation.

Modified classes are based on 2.6 of xwiki-core.jar :
com.xpn.xwiki.doc.XWikiAttachment
com.xpn.xwiki.api.Attachment

Modified template is based on 2.6 too :
changeavatar.vm

A basic ImageInfo object (inner class in XWikiAttachment, get images properties with javax.imageio)
is singletoned in XWikiAttachement. It is made availlable from templates via api.Atttachment

Note that in changeavatar.vm, error messages and max dimensions are hardcoded and tha the redirection if picture too big is done upon javascript