Fix gridlines for charts

Adds the gridlines back in, horizontally and vertically. Gridlines are
now clipped to be within the plot area of the chart, and line up with
the tick marks.

Also modified the gridlines() setter function to update whether or not
gridlines are visible for the chart.
Method renaming

Renames this.gridlines => this.showGridlines
Adds this.gridlines (again), returning a readonly array of all current
gridlines on the axis.
Adds tests for gridlines

Adds some new unit tests for gridlines.
Also defaults the gridline colour to the colour of the axis.
Remove redundant comment

We no longer comment out this.drawGridLines, so don't need the comment
explaining as such.
Merge pull request #70 from tpkelly/master

Fix gridlines for charts