Adds serializer format specific field names

Allows different field names to be used for different external formats.

Field names are specified using the `rename` field attribute, e.g:

    #[serde(rename(xml= "a4", json="a5"))]

Reverts #62

Addresses #61

src/ser/mod.rs

@@ -179,6 +179,10 @@ pub trait Serializer {
     fn visit_map_elt<K, V>(&mut self, key: K, value: V) -> Result<(), Self::Error>
         where K: Serialize,
               V: Serialize;
+
+    fn fmt() -> &'static str {
+        ""
+    }
 }
 
 pub trait SeqVisitor {