added sphere example

[grburgess]

This adds a note book with a walkthrough on how to do a UV mapping of a texture to a sphere

[coveralls]

Coverage increased (+0.1%) to 66.328% when pulling 764505a on grburgess:feature-sphere-example into fad27f9 on maartenbreddels:master.

[maartenbreddels]

Thanks, I have changed the examples, to be gallery style:
https://github.com/maartenbreddels/ipyvolume/pull/298/files

Would you mind doing sth similar? You can use 64c5a77 as an example.

See requirements_rtd.txt for the requirements for the docs

[grburgess]

Will do!