Updated layout for GH's new metadata support

See: https://github.com/blog/1797-repository-metadata-and-plugin-support-for-github-pages
Merge pull request #24 from jonalmeida/patch-1

Updated layout for GH's new metadata support
Use site.github.owner_url
Full URL
Kill gravatar_url
Update README
Add a link
Update gravatar site variable

Was not working for me with site.gravatar_url.  Noticed the different
variable usage (site.github.owner_gravatar_url) in layout.html and used
that.
Merge pull request #26 from iamgabeortiz/gh-pages

Update gravatar site variable
Use jekyll-sitemap plugin
Merge pull request #27 from pathawks/sitemap

Use jekyll-sitemap plugin