Many small bugfixes and features ...

I didn't want to make a pull request for every of these little bugfixes and features:

* There is a new optional parameter for "parse_url" to only fetch data from the given meta data. This is useful when page information is added automatically (like in the twitter addon) The default behaviour (of course) hadn't changed
* The structure of the title of oembedded links has changed. The previously used english words "by" and "on" are irritating for non english speaking users. This is changed to a structure without additional words.
* The "posted" value for shared content is interpreted as a relative value. This is a problem when the item cache is active - since this "6 minutes ago" will be stored as a value for the cache duration. So this value is disabled for systems with active cache.
* When diaspora user names contained the values "[", "]" or "'" then there was a problem with shared messages. These values are now encoded.
* previously, hooks where called only after their priority. Now they are sorted after priority and the file name. This has the advantage that they are ordered after their name at the settings page.
* And last but not least there were some SQL warnings left that had to be solved.