Refactor a bit

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

src/lib.rs

@@ -14,3 +14,25 @@ extern crate num_traits;
 pub use self::decoder::TIFFDecoder;
 
 mod decoder;
+mod error;
+
+pub use self::error::{TiffError, TiffResult};
+
+/// An enumeration over supported color types and their bit depths
+#[derive(Copy, PartialEq, Eq, Debug, Clone, Hash)]
+pub enum ColorType {
+    /// Pixel is grayscale
+    Gray(u8),
+
+    /// Pixel contains R, G and B channels
+    RGB(u8),
+
+    /// Pixel is an index into a color palette
+    Palette(u8),
+
+    /// Pixel is grayscale with an alpha channel
+    GrayA(u8),
+
+    /// Pixel is RGB with an alpha channel
+    RGBA(u8),
+}