Header tags and em Sass function

- Added a function to convert pixel font sizes to em automatically
  - @mpiotrowicz, used a slight variation of the one you pointed me to. Uses `em(20px)` rather than just `em(20)`
  - I think it makes it a bit more clear to understand what is happening with the unit attached
- Cleaned up use of `h1` and `h2` page title tags (as per recent comment on #90)
- Minor cleaning of padding/margins to use gutter rather than ems

@jholl @stevebosworth