Remove taxi_preprocessing_example?

[jbednar]

Copied from #564:

• [x] pillow was pinned to 4.3.0 on appveyor; need to test ds pil usage on windows. (where did problem show up - is it covered by unit tests + running the limited set of examples, which is what happens on appveyor?)
• [x] now we dropped the examples package, should move topics examples and dependencies to pyviz (maybe future PR? jbednar: yes and yes) [EDIT: Now https://github.com/pyviz/pyviz/issues/84]
• [x] now we dropped the examples package, how do users know to get holoviews - via docs? (holoviews is imported in user guide notebooks , but isn't a required dependency.) (jbednar: mention in install instructions, as long as it's just a couple of packages to install)
• [ ] how does taxi_prepreprocessing_example relate to download_sample_data.py? And was it intended for running as a script (because that's what it is now...) (it's an example, and isn't needed for any other purpose as the output of it is what we are providing for download)
• [x] To do for CB: Travis file modifications before merge, once testing on this branch is done.
• [x] restore doc/_static files now decided they will not be in examples/assets
• [x] CB got ShapefileException: Unable to open ../data/cb_2015_us_cd114_5m.dbf or ../data/cb_2015_us_cd114_5m.shp when running examples on linux (but file's hash matched JAB's).
• [x] remove requirements.txt (dependencies are in setup.py)
• [x] TODO in .travis file about switching from c-f to defaults
• [x] update installation docs