Split into a library and many modules

Wow, what a mess. Why was I making one giant file to contain everything?
I'm splitting it out into several files and taking another crack at
completing the decoder.

src/error.rs

@@ -0,0 +1,8 @@
+
+#[derive(Debug)]
+pub enum DecodeError {
+    Magic,
+    Channels,
+    ColorSpace,
+    EarlyIteratorExhaustion,
+}