Fix bug where end ticks were not displaying for certain data values in sparklines. 

As follows: 

* If showStartAndEndTicks is set to true, add option in y-axis.js to provide explicit tickValues to D3.
* Rename some confusing existing property names.
* Write a new test for y-axis. 

See individual commits for full details. 

 Pivotal bug:
 https://www.pivotaltracker.com/s/projects/911874/stories/65885302