ENH: nightwatch (again)

[hainm]

Damn, finally I can make notebook test with chromedriver works again.
Very tricky (need to use correct chromedriver version (in this case, v2.30)).

Example

$ nightwatch 
Starting selenium server... started - PID:  17011

[Render Image] Test Suite
=============================

Running:  nglview/tests/notebooks/api/render_image.ipynb
url http://localhost:8889/notebooks/nglview/tests/notebooks/api/render_image.ipynb
 ✔ Passed [ok]: Check that python has no error
 ✔ Passed [ok]: Check that python has no error
 ✔ Passed [ok]: Check that python has no error
 ✔ Passed [ok]: Check that python has no error
 ✔ Passed [ok]: Check that python has no error
 ✔ Passed [ok]: Check that python has no error