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/de/mod.rs

@@ -113,6 +113,10 @@ pub trait Deserializer {
     {
         self.visit(visitor)
     }
+
+    fn fmt() -> &'static str {
+        ""
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////////

src/json/de.rs

@@ -463,6 +463,11 @@ impl<Iter> de::Deserializer for Deserializer<Iter>
             Err(self.error(ErrorCode::ExpectedSomeValue))
         }
     }
+
+    #[inline]
+    fn fmt() -> &'static str {
+        "json"
+    }
 }
 
 struct SeqVisitor<'a, Iter: 'a + Iterator<Item=io::Result<u8>>> {

src/json/ser.rs

@@ -256,6 +256,11 @@ impl<W, F> ser::Serializer for Serializer<W, F>
         try!(self.formatter.colon(&mut self.writer));
         value.serialize(self)
     }
+
+    #[inline]
+    fn fmt() -> &'static str {
+        "json"
+    }
 }
 
 pub trait Formatter {

src/json/value.rs

@@ -571,6 +571,11 @@ impl ser::Serializer for Serializer {
 
         Ok(())
     }
+
+    #[inline]
+    fn fmt() -> &'static str {
+        "value"
+    }
 }
 
 pub struct Deserializer {
@@ -677,6 +682,11 @@ impl de::Deserializer for Deserializer {
             None => Ok(value)
         }
     }
+
+    #[inline]
+    fn fmt() -> &'static str {
+        "value"
+    }
 }
 
 struct SeqDeserializer<'a> {

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 {