Make Buf and BufMut usable as trait objects (#186)

- All the `get_*` and `put_*` methods that take `T: ByteOrder` have
  a `where Self: Sized` bound added, so that they are only usable from
  sized types. It was impossible to make `Buf` or `BufMut` into trait
  objects before, so this change doesn't break anyone.
- Add `get_n_be`/`get_n_le`/`put_n_be`/`put_n_le` methods that can be
  used on trait objects.
- Deprecate the export of `ByteOrder` and methods generic on it.

Fixes #163

tests/test_buf.rs

@@ -33,15 +33,15 @@ fn test_get_u8() {
 #[test]
 fn test_get_u16() {
     let buf = b"\x21\x54zomg";
-    assert_eq!(0x2154, Cursor::new(buf).get_u16::<byteorder::BigEndian>());
-    assert_eq!(0x5421, Cursor::new(buf).get_u16::<byteorder::LittleEndian>());
+    assert_eq!(0x2154, Cursor::new(buf).get_u16_be());
+    assert_eq!(0x5421, Cursor::new(buf).get_u16_le());
 }
 
 #[test]
 #[should_panic]
 fn test_get_u16_buffer_underflow() {
     let mut buf = Cursor::new(b"\x21");
-    buf.get_u16::<byteorder::BigEndian>();
+    buf.get_u16_be();
 }
 
 #[test]