Improve readibility.

- Increase body font size to 16px
- Make darker body color (as the background is not white, we missed constrast)
- Increase line height in order to follow font size increase
Improve look n feel pre tag
Increase margin between <p> <ul> <hX> and <pre>
Same font for <pre> and <code>
Merge pull request #41 from tyx/fix/improve-readibility

Improve readibility.