adding phpdoc blocks in addressbook classes (one more backend to go)

adding phpdoc blocks in addressbook classes (one more backend to go)
adding global file based address book support.
global_file backend removed (code reuse)
administrator plugin still need some mods, but it is dark outside and i'll
finish it later.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@8317 7612ce4b-ef26-0410-bec9-ea0150e637f0