Factor tags into own module

Tag representations are not decoder- or encoder-specific, so should live
in their own module accessible to both for reuse. The decoder::ifd
module remains since this module is mainly dedicated to decoding the
IFD.

src/error.rs

@@ -3,8 +3,8 @@ use std::fmt;
 use std::io;
 use std::string;
 
-use decoder::ifd::{Tag, Value};
-use decoder::{CompressionMethod, PhotometricInterpretation, PlanarConfiguration};
+use decoder::ifd::{Value};
+use tags::{CompressionMethod, PhotometricInterpretation, PlanarConfiguration, Tag};
 use miniz_oxide::inflate::TINFLStatus;
 use ColorType;