Explicitly set sans-serif to Arial.

Font defaults querying fails on Chrome, causing nodes
be rendered in Times where there should be sans-serif
Helvetica. Setting explicit font fixes this as defaults
do not be resolved.

Until this bug is fixed in Chrome stable, we must use
explicit font, or Mac users have degraded readability.

The font resolving does not affect actual spacing, so
the pages don't just use Times, but are all sorts of
distorted; Times is squeezed where Helvetica would have
been.

Fixes #56