Add vanity import name (#107)

* api: add doc.go and vanity custom import name

* experimental/streaming/exporter: add doc.go & vanity custom import name

* experimental/streaming/exporter/*: add vanity custom import name

* golangci.yml: fix local-prefixes to go.opentelemetry.io

* api,sdk: run `goimports -w -local go.opentelemetry.io`

api/core/doc.go

@@ -0,0 +1,15 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package core // import "go.opentelemetry.io/api/core"