Add about_link, feature captions, and feature alt

The following additions and chagnes were made to the navigation sidebar,
and to the feature images in the page, post, and default (hom page)
layouts:

* Change "Learn More" link in Navigation->About from About.me site to
  any path/url
  - configuration changed from `site.aboutme_user` to `site.about_link`
  - `site.about_link` should be absolute, e.g. either `/my_page` or
    `http://mysite.com`
* Add optional image captions to feature images on pages, posts, and
  default (home page) layout
  - Use `page.image.caption` on page source file
  - Image caption is displayed on top of image credit
  - If only one exists, only that one is displayed
  - If neither exists, the credit bar isn't shown
* Add customizable alt text to feature images on pages, posts, and
  default (home page) layout
  - Use `page.image.featurealt` on page source file
  - If no custom alt text is given, the page title is used
  - If custom alt text is given, it is placed in square brackets as per
    accessibility guidelines
* Add optional subtitles to page layout
  - Use `page.subtitle` on page source file
  - If it isn't specified, it is ignored
modifications to _config.yml

There were a few errors with _cofig.yml, and the previous commit
required some new fields

* Add: `about_link`
* Remove: `googleplus_user`
* Modify: `disqus_short_name`
* Add: `google_analytics_tracking_id`
* Remove: `aboutme_user`