Resove issue #18 (Default <head> on no database)

Get $head from database only if $DB->connected and provide fallback.

Only load $pages if $DB->connected too.
Updated documentation
Do not make database queries if connection fails

Resolves issue #14
Added polyfill for CSS.escape()
Updated @supports links
Updated `<img>` for README & updated `img-src` in csp.ini
Consistency and more flexbox