1) disjunction and conjunction queries now support a
"query string mode". By default they do not operate
in this mode. When in this mode, any disjunct/conjunct
which evaluates to MatchNone searcher, will be removed
from the disjunction/conjunction. If the query ends
up with NO conjuncts/disjuncts, it will itself
return the MatchNone seacher.
2) boolean query also supports a query string mode. when in
this mode, the Must, Should and MustNot searchers are all put
into query string mode.
3) rewriting of negation only queries (like -foo) now take into
account the rewriting rules above, and those are handled first.
this means that we rewrite correctly in case of +stoword -foo
4) the empty query string is now valid, and returns 0 hits.
previously this was considered a validation error.
