Fix for multiple negative words queries

[phurni]

When querying with multiple negative words, I think that the expected behaviour is to match documents without any of them, not all of them.

Example: dog -cat -bird should match all documents with dog but without cator bird

Currently it matches all documents with dog without catAND bird

[dougal]

So if we have four documents:

• 1 - cat dog bird
• 2 - cat dog
• 3 - cat bird
• 4 - cat

Then the search: cat -dog -bird Should return documents 2, 3 and 4?

[phurni]

@dougal No, my understanding of this query is: documents with cat but without dog or without bird. So it should yiled only document 4.

[phurni]

Ok, since the semantics of those kind of queries are open to interpretations, I took a different approach. I refactored the SearchIndex to be able to default to OR between query terms.

Forget about this PR, I'll send a new one with the new code which will not break existing queries.