Redirect dropbox URLs.

Fixes #141.
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.
Merge remote-tracking branch 'upstream/master' into qa