layouts: restore compatibility with Hugo < 0.18

header: support for static avatar.

header: add Gravatar support.

Fixes params get for Hugo 0.18

layouts: access DateForm and DateFormFull paramx as string.

This fix the usage of this theme with Hugo 0.18-dev

images: Update thumbnail image

.container .content CSS and .post-item HTML fixes

* .container .content for icons should not be 100%
  to help with centering.
* Use span for .post-item internals, otherwise Firefox
  messes up at smaller screen width with the list bullet
  and title on separate lines.
* README wording

Fix #17

RSS support

Also, add a link to demo web page.

Fixes #7

Add highlight js for syntax highlighting

Also, improve README.md wording on config.toml.

Fix back link

- Change color from orange to blue; improve colors elsewhere
- Change `div.section` to `section`
- Rename  `posts` directory to `blog`
- Add ability to specify extra CSS files in `config.toml`
- Removed `WebFontsFile` feature from `config.toml`
- Remove the initials displayed on top right of single post pages
- Update example site