G4 support

G4 tiff read/write support.

Current status: Mostly working.

Works:

* There are tests for file and stringio based image reading
* There are tests for small and large images (there's an internal break at 64k due to a buffer size)
* There's a sanity test for writing that we're getting back what we write out. 
* Redistributable images for the tests -- lena and a scan of a US government document. 

What's not working:

* A small lena image is failing to read within libtiff. There's a currently disabled test for it. I'm going to dig further, as I don't see what's different about this image that is causing it to fail.