using relationships

[MhdSyrwan]

we should use ElggRelationship to link entities instead of using meta data
this fix should reduce query cost

[isstaif]

This is the same issue as #17.

I found that the high cost problem is not caused by the absence of ElggRelationships. It is rather because of "diff" fetching process. If you notice we are making a separate Wikimate request for each wikiactivity.

https://github.com/wikitechie/socialwiki/blob/master/lib/socialwiki.php#L92