Add support for Webhook note events

CHANGELOG

app/controllers/projects/hooks_controller.rb

app/models/hooks/project_hook.rb

app/models/hooks/service_hook.rb

app/models/hooks/system_hook.rb

app/models/hooks/web_hook.rb

app/services/notes/create_service.rb

app/views/projects/hooks/index.html.haml

db/migrate/20150516060434_add_note_events_to_web_hooks.rb

db/schema.rb

doc/web_hooks/web_hooks.md

lib/api/project_hooks.rb

spec/models/hooks/project_hook_spec.rb

spec/models/hooks/service_hook_spec.rb

spec/models/hooks/system_hook_spec.rb

spec/models/hooks/web_hook_spec.rb

spec/services/notes/create_service_spec.rb