Add copy button to docs

[rsokl]

Upgrade nbsphinx to 0.6

Add https://github.com/choldgraf/sphinx-copybutton capability

[Euquimides]

Hi! I added a PR for this issue #447. It was mentioned that MyNN was going to be merged with MyGrad as in here davidmascharka/MyNN#72. So I thought that maybe I could help since I find this project interesting. Thank you for your attention!