Issue/36 (#559)

* Added explanation

* Explain

* Splitting weight and idf

* Added comments

Closes #36

src/query/empty_query.rs

@@ -1,6 +1,7 @@
 use super::Scorer;
-use query::Query;
+use query::explanation::does_not_match;
 use query::Weight;
+use query::{Explanation, Query};
 use DocId;
 use DocSet;
 use Result;
@@ -32,6 +33,10 @@ impl Weight for EmptyWeight {
     fn scorer(&self, _reader: &SegmentReader) -> Result<Box<Scorer>> {
         Ok(Box::new(EmptyScorer))
     }
+
+    fn explain(&self, _reader: &SegmentReader, doc: DocId) -> Result<Explanation> {
+        Err(does_not_match(doc))
+    }
 }
 
 /// `EmptyScorer` is a dummy `Scorer` in which no document matches.