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.
I removed the Unicode literal to support py3.2 testing on Travis.  Technically Crhistophe only supports 2.7 and 3.3 (or 3.4, I'm on my phone).