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

**(DO NOT MERGE YET. FOR DISCUSSION ONLY.)**

Activate it by setting in LocalSettings.php:
  $wgSearchType = 'SMW\\SearchSMW';

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.

**Todo:**
* introduce a variable $smwgFallbackSearchType to be able to fall back to
  something different than the default search engine (basically a replacement
  for $wgSearchType that is now used for SMW\SearchSMW)
* Tests
* README

**Any comments (in particular regarding architecture) welcome.**