Add Whoosh as an alternate search backend

I've added Whoosh (https://bitbucket.org/mchaput/whoosh/wiki/Home) as a search backend to the search module. 

Few notes:

- I'm not a Whoosh expert. There might be ways to improve search results, someone with more Whoosh experience should check my code
- I'm not quite happy with the large amount of in-function imports (especially in WhooshSearch.\_\_init\_\_)
- I use LanguageAnalyzer per default with a fall-back to SimpleAnalyzer in case the desired language is not supported by Whoosh. Should Realms display a warning if that's the case? If so, what's the preferred way to do so?