Create subheader style.

[nicwinn]

Sometime's headers can get out of control. To avoid the temptation of using a smaller header than what would be correct we can add a subheader class. It can scale things back a bit and keep everything semantic.
http://foundation.zurb.com/docs/components/typography.html#subheaders

Example

Using <small>

I ended up wrapping the text in <small></small> which created extra work and didn't seem as good as adding a class.