Merge branch 'master' into travis/performance

api/r0/upload.go

api/unstable/local_copy.go

cmd/import_synapse/main.go

config.sample.yaml

controllers/download_controller/download_resource_handler.go

...rollers/maintenance_controller/maintainance_controller.go

controllers/preview_controller/preview_resource_handler.go

...ollers/thumbnail_controller/thumbnail_resource_handler.go

controllers/upload_controller/upload_controller.go

storage/datastore/datastore_ref.go

storage/datastore/ds_s3/s3_store.go