This PR allows for multi-line labels, as per Slack discussion..

Usage: If a label is an `array` as opposed to a `string` i.e. `[["June","2015"], "July"]` then each element is treated as a seperate line. The appropriate calculations are made to determine the correct height and width, and rotation is still supported.

view samples/line-multiline-labels.html to see it working.

On branch multiline_labels
Changes to be committed:
	modified:   docs/03-Line-Chart.md
	new file:   samples/line-multiline-labels.html
	modified:   src/core/core.helpers.js
	modified:   src/core/core.scale.js