Added automatic type scaling based on modular scales

Using modular scale principles (http://alistapart.com/article/more-meaningful-typography), I added the option for automatic heading scaling.

This still uses the typecsset base functions/mixins, but allows the user to create a typographic scale for slightly more harmonious type scaling.

Some of this might be able to be cleaned up, but the functions work as suggested. 

Future possibility: Make the modular-scale function available via a mixin (or variable list) to allow users to access their scale outside of just headings.