fetch gpg signature badges by ajax

app/assets/javascripts/dispatcher.js

app/assets/javascripts/gpg_badges.js

app/controllers/projects/commits_controller.rb

app/helpers/commits_helper.rb

app/models/commit.rb

app/views/projects/commit/_ajax_signature.html.haml

app/views/projects/commit/_signature_badge.html.haml

app/views/projects/commits/_commit.html.haml

app/views/projects/commits/show.html.haml

config/routes/repository.rb

spec/features/commits_spec.rb