Implement Commit Status API

CHANGELOG

app/models/ability.rb

app/models/ci/build.rb

app/models/ci/commit.rb

app/models/commit.rb

app/models/commit_status.rb

app/models/generic_commit_status.rb

app/services/ci/create_commit_service.rb

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

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

app/views/projects/commit_statuses/_commit_status.html.haml

...rate/20151008123042_add_type_and_description_to_builds.rb

.../20151008130321_migrate_name_to_description_for_builds.rb

db/schema.rb

doc/api/commits.md

lib/api/api.rb

lib/api/commit_statuses.rb

lib/api/entities.rb

lib/ci/api/entities.rb

spec/factories/ci/builds.rb