Elgg dokuwiki integration

Adds wikis for groups, and integrates the user and permission system of elgg.

- Although the goal is to be able to use vanilla dokuwiki with maybe some extensions,
  at the moment we provide a slightly modified version.
- doku.php globals are not known, and had to be redeclared.
- $_REQUEST not defined... dont know the reason but its something of elgg. -> yes elgg is mangling it... **had to comment it on elgg core**
- acl is hardcoded, added a getACL method on the auth class to be able to generate
  the acl on the fly... also setACL and modified the admin plugin to be able to use the auth backend.
- uses an 'elgg' auth module, and a special dokuwiki template.
- dokuwiki defines the javascript $ symbol, which conflicts with elgg usage of jquery, so had to change all usage inside dokuwiki (just a few instances).

- copy into the mod folder as dokuwiki
- only for elgg <= 1.7.1: you need to modify .htaccess, adding &%{QUERY_STRING} to the pg/ rules.
  after modifying they should look like this:

RewriteRule ^pg\/([A-Za-z0-9\_\-]+)\/(.*)$ engine/handlers/pagehandler.php?handler=$1&page=$2&%{QUERY_STRING}
RewriteRule ^pg\/([A-Za-z0-9\_\-]+)$ engine/handlers/pagehandler.php?handler=$1&%{QUERY_STRING}

this change is already in elgg trunk, but 1.7.1 didn't have it yet.

- elgg plugin uses GPLv2. dokuwiki and mods (all under lib/dokuwiki) go with original license to original copyright holders.

You can follow code changes or submit tickets at: https://rhizomatik@bitbucket.org/rhizomatik/elgg_dokuwiki/

- Maybe just one big wiki for the whole site using namespaces?