added build step for ebook
use \sigma instead of raw unicode character in math mode, as it
does not render in the PDF otherwise.
Adjust layout

- Reduced width of layout
- Less spacing between paragraphs
set Luxi Mono and Heuristica (Utopia) as the default fonts for
monospace and main text respectively for PDF output.

Added notes on required packages to README
Merge pull request #2 from soc/layout

blueprint CSS layout modified to 18-column from 24-column, spacing between paragraphs reduced by 1em