Move hound crate to map::wav module and add cfg decorator.

src/lib.rs

@@ -1,4 +1,3 @@
-extern crate hound;
 extern crate instrument;
 extern crate pitch_calc as pitch;
 extern crate sample;

src/map.rs

@@ -151,8 +151,10 @@ impl<F> Map<F>
 }
 
 
+#[cfg(feature="wav")]
 mod wav {
-    use hound;
+    extern crate hound;
+
     use map;
     use pitch;
     use sample;