Signed value support

Add support for SBYTE, SSHORT, SLONG, SRATIONAL as per the Tiff 6.0 spec
Add support for multiple BYTE

src/error.rs

@@ -33,6 +33,7 @@ pub enum TiffFormatError {
     RequiredTagNotFound(Tag),
     UnknownPredictor(u32),
     UnsignedIntegerExpected(Value),
+    SignedIntegerExpected(Value),
 }
 
 impl fmt::Display for TiffFormatError {
@@ -51,6 +52,9 @@ impl fmt::Display for TiffFormatError {
             UnsignedIntegerExpected(ref val) => {
                 write!(fmt, "Expected unsigned integer, {:?} found.", val)
             }
+            SignedIntegerExpected(ref val) => {
+                write!(fmt, "Expected signed integer, {:?} found.", val)
+            }
         }
     }
 }