If a post has a description, use it in the XML data

I noticed that the system was putting the entire post body in the description.  It is more desirable to place just the description of the post in there.  You can now add the description: yaml markup to your posts to have a more clean feed XML.  Alternatively, if you do not specify a post description, the page title will be used (instead of the post body).
defaulting description to excerpt if exists and removing layout none which was throwing warnings on build time
adding logic to display the appropriate excert, content or site description in all feeds files and removing layout: none from everywhere to prevent warnings on builds
updating to use post excerpt if exists but default to entire post content
Merge pull request #11 from integrii/master

Merging per:  https://github.com/snaptortoise/jekyll-rss-feeds/pull/11