Pull out gridlines to new class

Pulls out the drawing code for gridlines into its own class, ready to
add some styling methods to it.

Also updated the test to remove the chart before doing the expect() call
to make sure it gets removed properly.
Allow custom line width and colour

Adds the lineWidth/lineColor getter/setters, and hooks them in to the
gridline drawing.
Merge pull request #73 from tpkelly/master

Refactor gridlines