add slack notifications for wiki pages

CHANGELOG

app/controllers/projects/services_controller.rb

app/controllers/projects/wikis_controller.rb

app/models/hooks/project_hook.rb

app/models/project_services/slack_service.rb

...odels/project_services/slack_service/wiki_page_message.rb

app/models/service.rb

app/models/wiki_page.rb

app/services/wiki_pages/base_service.rb

app/services/wiki_pages/create_service.rb

app/services/wiki_pages/update_service.rb

app/views/shared/_service_settings.html.haml

db/migrate/20160227120001_add_event_field_for_web_hook.rb

db/migrate/20160227120047_add_event_to_services.rb

db/schema.rb

.../project_services/slack_service/wiki_page_message_spec.rb

spec/models/project_services/slack_service_spec.rb