resolves #879 switch to Open Source fonts in default stylesheet

- Open Sans (Light) for section and block titles
- Noto Serif for body text
- Droid Sans Mono for code and literal text
- fallback to DejaVu fonts if fonts aren't loaded
- control loading of fonts using webfonts attribute
- align colors to color palette
- remove background color and border on inline monospaced text
- use style class for admonition icons that won't conflict with FontAwesome
- remove several unused style declarations
- add background color to toc sidebar
- fix margins in sidebar toc
- don't underline links in TOC
- fix padding in labeled list terms
- fix spacings in blockquote
- add program language label for "Go"
- fix padding around listing block content