Use PyPI Tifffile or Fall back on Local Version

This uses the new [tifffile](https://pypi.python.org/pypi/tifffile) package if possible, or one included in `skimage.external.tifffile`.  
I also broke the travis script into three pieces, so we can `travis_retry` the optional package installs.  Also, we pull from the new "http://travis-wheels.scikit-image.org/" wheelhouse.
Does this mean that tifffile doesn't support unicode strings? What happens on Py3 here?