Adds a search engine to allow semantic queries directly from MW's standard search

This search engine will try to interpret the search term as a semantic query.

If succesful, the pages according to the query will be returned.
If not it falls back to the default search engine.

* Activate it by setting in LocalSettings.php: $wgSearchType = 'SMWSearch';
* Use $smwgFallbackSearchType to set the fallback search engine to be used if the
  search term is not a valid SMW query
Merge pull request #450 from s7eph4n/master

Adds a search engine to allow semantic queries directly from MW's standard search