Use PyPI Tifffile

This uses the new [tifffile](https://pypi.python.org/pypi/tifffile) package instead of including it in our repo.   The correct version of `numpy` must be installed prior to installing `tifffile`.  Ideally, `tifffile` should be included in http://wheels.scikit-image.org/ so we can install all of our `requirements.txt` files from there.