Update contributor counter to GitHub API v3
Update contributor counter to GitHub API v3

The GitHub API paginates the response to the /contributors endpoint,
including a maximum of 100 elements per page.
To obtain the number of contributors with a single request, we ask the
API to respond with 1 element per page, and parse the `Link` response
header for the number of pages.

Fixes #20.

rafaelbiriba/caskroom.github.io,elovelan/caskroom.github.io,elovelan/caskroom.github.io,caskroom/caskroom.github.io,rafaelbiriba/caskroom.github.io,caskroom/caskroom.github.io