Fix bug in seo_metadata

Update #15
Add support for og:image and twitter:image using article.social_media_image

Update #15
Add support for og:image and twitter:image for site's home page. Also let user set a default image for the purpose

Update #15
Move SITE_LICENSE to _defaults
Move SITE_DESCRIPTION to _defaults
Move MAILCHIMP_FORM_ACTION to _defaults
Move SITESUBTITLE to _defaults
Fix a check in home page social media tags

Update #15
Move LANDING_PAGE_ABOUT to _defaults
Move PROJECTS to _defaults
Move USE_FAVICON to _defaults
Update release notes
Add support for multi_parts plugin

Fix #95
Rename SOCIAL_MEDIA_IMAGE to FEATURED_IMAGE

Update #15
Add fontawesome class names of social sites icons to the existing CSS rules
Update release notes and thanks file
Merge branch 'master' into calf
update multi part