Adds label_color as an option to line charts.

Hi Josh,

This commit adds a setting for the label colour in line charts, allowing users to override the default black label with a hex colour when creating a line chart. This is done by adding a "color" method to the LabelSet class which changes the @color property, and by adding a "label_color" @option to the "draw_x_label" method in the LineChart class.

Changing the "label_color" option for a line graph changes the color of all labels, like so:

    abtest.chart = new Charts.LineChart("graph", {
        show_grid: true,
        label_color: '#f00', // Makes all labels red
        max_y_labels: 5,
        max_x_labels: 20,
        y_label_size: 12,
        x_label_size: 12,
    });

If no label colour is provided the default colour '#333' is used.