Refactor a bit

Rename the Image* types to Tiff*
Extract the "common" types to the top level.

src/decoder/stream.rs

@@ -1,7 +1,6 @@
 //! All IO functionality needed for TIFF decoding
 
-use std::io;
-use std::io::{Read, Seek};
+use std::io::{self, Read, Seek};
 use byteorder::{ReadBytesExt, BigEndian, LittleEndian};
 use lzw;