Reference Lines & Frames for Y-Axis; Bug Fixes

Optionally draw horizontal reference lines on the graph for each y-axis
label. Additionally, optionally draw a line to the right of the y-axis
to
separate the graph and the y-axis. Fixed bugs with the autoScaleYAxis
property when set to NO. Removed stepValueBetweenYLabelsOnLineGraph
delegate method use numberOfYAxisLabelsOnLineGraph instead. Removed
enableYAxisLabelOffset.