add the plugin to extract representative image for article and assign to an article's property
change image property name to featured_image
update Readme.md
Merge pull request #200 from khuevu/master

Add a plugin to extract representative image for an article
Fix Sitemap generation for non-file urls.

Fixes getpelican/pelican-plugins#168.
Merge pull request #203 from adworacz/fixSitemapUrlGeneration

Fix Sitemap generation for non-file urls.
adding markdown specific instructions to gravatar readme
Merge pull request #205 from TheRealBill/master

Add Markdown-specific instructions to Gravatar plugin README
Better twitter share url.

http://dev.twitter.com/pages/intents
Add LinkedIn share URL.