Pull examples from https://github.com/wlandau/drake-examples

[wlandau]

The examples take up 2.1 MB out of the 5 MB allowed by CRAN. In the interest of growing the example base, I have decided to host examples from https://github.com/wlandau/drake-examples and host zipped copies on GitHub pages for download. The drake_example() and drake_examples() functions should download files from https://wlandau.github.io/drake-examples.

[wlandau]

The docs also need to refer to https://github.com/wlandau/drake-examples rather than https://github.com/ropensci/drake/tree/master/inst/examples.