Adding StackOverflow social icon and support for Disqus post comments

Conflicts:
	_config.yml
adds disqus_shortname
fix post-image-feature images link in post.html layout
Use Formspree instead of Brace Forms

From an email from Brace Forms:

> Brace.io is closing down, and Brace Forms has been open sourced. You have the option to move to formspree.io, a new service based on the open sourced code and hosted by Assembly.com. Check it out at http://assembly.com/formspree. Alternatively you can host your forms using your own server.
>
> WHAT YOU SHOULD DO:
>
> You must change your form URL to formspree.io, or your own hosting, and re-confirm by January 19th. To do this:
>
> 1. Change the action URL on your form from "http://forms.brace.io/your@email.com" to "http://formspree.io/your@email.com", or use your own server. Instructions for self hosting can be found at http://github.com/braceio/forms.
>
> 2. Submit the form once, and click "confirm" in the generated confirmation email.
Make blockquotes prettier on small-screen devices
Prettier table styles from Bootstrap

Ported from Bootstrap Sass. Not all styles are included as not all styles are needed.
Conflicts:
	_sass/_main.scss
Use redcarpet as the default markdown engine

It is closer to the GitHub syntax, and is what most people using GitHub and Jekyll are most familiar with.

Also use some extensions:

- smart: Enable SmartyPants. kramdown had this on by default
- tables: Enable table parsing. kramdown had this on by default
- with_toc_data: Enable heading anchors. All markdown documents on GitHub have this on, so why not here too?
Use an identifier for Disqus

Disqus creates a new thread for comment from a different URL. This means that the page visited from HTTP have a different thread than the one on HTTPS. Work around this using `page.id` as `disqus_identifier`. `page.id` is guaranteed to be unique by Jekyll:

> ## Page Variables
>
> VARIABLE | DESCRIPTION
> ----|----
> `page.id` | An identifier **unique** to the Post (useful in RSS feeds). e.g. `/2008/12/14/my-post`

From http://jekyllrb.com/docs/variables/
Add forgotten semicolon

For consistency as well as need. There is a parenthesis after the string.
adds missing closing bracket due to faulty merge