New front page header

Photograph by @mkhandekar

Closes #37
Small header style correction
Fix the build

* Adds alt attributes to blog images.
* Changes decorative down-arrow image to be specified in css.
* Fix link to mask mag
* Fix hash-link in flock's legal policy
* Switched to using a rake task in order to configure htmlproof to
  accept external links with self-signed certs. (The command line
  htmlproof doesn't expose all available options).
* Configure htmlproof to allow linking to # (the null anchor)
Signal blog post

// FREEBIE
Fix failing build due to missing alt tags
Merge pull request #38 from ukd1/feature/fix-build

Fix failing build due to missing alt tags
Add AppStore smartbanner to front page

Or any page with `smartbanner: true` in its yaml frontmatter.
Merge pull request #44 from WhisperSystems/smartbanner

Add AppStore smartbanner to front page
Add smartbanner to Signal blog post.

// FREEBIE
Adds responsive about page, headshots, and link to about page to nav
Fixes wrap issue, adds links to the about org paragraph
Fix nav link, add arrow, reduce whitespace