Add meta tags in notebook template header.
Fixes #164
Fixes #18
Templatize author, description only within layout.
Switch to hosted ipynb icon
twitter:title should be non-empty
Replace template with plain string
Redirect dropbox URLs.

Fixes #141.
allow to use files/ on gists

closes gh-155
Merge pull request #156 from Carreau/fix-gist-files

allow to use files/ on gists
Merge pull request #168 from rgbkrk/dropbox

Redirect dropbox URLs.
compact dropbox rewrite + test
Merge pull request #171 from Carreau/compact-dropbox

compact dropbox rewrite + test
Merge pull request #167 from rgbkrk/twitter_card

Meta tags for twitter, description, author.
add github icons
Merge pull request #172 from Carreau/fa-icon

add github icons