Updated Modular Scale function to have defaults

- Added default value
- Added default ratio
- [Updated docs](https://github.com/thoughtbot/bourbon/pull/439)

Collaboration with @whmii 
It's pretty logical that one function would have a dependency on another. I am surprised that the `em()` function is not the first thing on there. We may want to make a distinction between *system functions* that are widely used by mixins and other functions and those that are *feature functions* that provide functionality but are not required by other portions of Bourbon.