Load DNGs to ndarray

Add tiffutils.load_dng(), which reads a DNG, and loads its contents into
an ndarray, which is returned.

References #292
Update README

Mention the ability to load DNGs, and fix a few typos.
load_dng: return CFA pattern

Along with image array, return the CFA pattern constant of the image's
CFA pattern.  Without this information, the image cannot be saved back
properly after being read.