Adjust secondary headings

Two small adjustments to the secondary heading styles:

1. At the moment they only work for x-large headings. I've added one for large headings as well.

2. The spacing between the secondary and main headings was very tight:

![image](https://cloud.githubusercontent.com/assets/1590604/4107399/65e8529e-31c7-11e4-8ea7-96a36dba901e.png)

I've switched to the 'heading' mixins which seems to have done the trick:

![image](https://cloud.githubusercontent.com/assets/1590604/4107403/76ee98dc-31c7-11e4-948a-fd97ea4634d5.png)