Use consistent line height and heading formula.

* Add a LESS var for base-line-height.
* Tweak heading formula to prove tighter wrapping for smaller `h*`, but not too spacey for larger `h*`.
Add class to provide strong brand font option.
Provide a simple brand font style.
Merge pull request #519 from smeranda/head-strong

Presentation adjustments for headings, impact text.