Add legend for line charts

# Line chart legends

This pull request adds an option to show a legend for a line chart. The legend is optional, but if enabled requires the `title` field to be defined in the dataset:

_Example dataset with showLegend_

	var data = {
		labels : ["January","February","March","April","May","June","July"],
		datasets : [
			{
				fillColor : "rgba(220,220,220,0.5)",
				strokeColor : "rgba(220,220,220,1)",
				pointColor : "rgba(220,220,220,1)",
				pointStrokeColor : "#fff",
				data : [65,59,90,81,56,55,40],
				//Optional: Add this if you want to show a legend
				title : "Foo"
			},
			{
				fillColor : "rgba(151,187,205,0.5)",
				strokeColor : "rgba(151,187,205,1)",
				pointColor : "rgba(151,187,205,1)",
				pointStrokeColor : "#fff",
				data : [28,48,40,19,96,27,100],
				//Optional: Add this if you want to show a legend
				title : "Bar"
			}
		]
	}
	
This produces something like this:

![](https://www.evernote.com/shard/s229/sh/f75fb946-2f89-4e5d-9014-bd9645bd52c5/c6964ff8f4efe07dafb1abdd64827ccd/deep/0/Line%20Chart.png)

The pull request also includes updates to documentation and samples.