Example: How to override time units label formatter

[RichardLitt]

In Rickshaw.Features.Time.js, we have different units.

{
	name: 'second',
	seconds: 1,
	formatter: function(d) { return d.getUTCSeconds() + 's' }
}

It would be great if we could have an example explaining how to override the formatter for i18n. Right now, m, s, and ms are all hardcoded - but these will change for other languages.

Related to #115 and #485.