Adds label_color as an option to line charts.

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 overriding the LabelSet's "Draw" method for Y axis labels, and by adding an @option to the "draw_x_label" LineChart class.

If no label colour is provided the default colour '#333' is used.
Adds axis names to line charts

This commit adds axis names to line charts, allowing you to show labels with
the names underneath the graph. You can name graph axis by doing the following:

	chart = new Charts.LineChart('graph', {
		y_axis_name:  'Conversion Rate',
		x_axis_name:  'Date',
		axis_name_color: '#aaa',
		axis_name_size:  10,
		x_padding: 60,
	});

There are options for the axis names, the axis font colour and the axis text
size to differentiate these from the labels for data values.

Note that right now you'll ahve to tweak X padding to work with your text size
for the Y axis.
Merge pull request #7 from tonyhb/develop

Adds label_color as an option to line charts.