G4 Tiff 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.  

I'd like to get this pulled in if possible to run it through the gauntlet of travis testing on all the other environments that I don't have setup.