api: Implement bucket notification. (#2271)

* Implement basic S3 notifications through queues

Supports multiple queues and three basic queue types:

1. NilQueue -- messages don't get sent anywhere
2. LogQueue -- messages get logged
3. AmqpQueue -- messages are sent to an AMQP queue

* api: Implement bucket notification.

Supports two different queue types

- AMQP
- ElasticSearch.

* Add support for redis

vendor/gopkg.in/olivere/elastic.v3/CONTRIBUTING.md

@@ -0,0 +1,40 @@
+# How to contribute
+
+Elastic is an open-source project and we are looking forward to each
+contribution.
+
+Notice that while the [official Elasticsearch documentation](https://www.elastic.co/guide/en/elasticsearch/reference/current/index.html) is rather good, it is a high-level
+overview of the features of Elasticsearch. However, Elastic tries to resemble
+the Java API of Elasticsearch which you can find [on GitHub](https://github.com/elastic/elasticsearch).
+
+This explains why you might think that some options are strange or missing
+in Elastic, while often they're just different. Please check the Java API first.
+
+Having said that: Elasticsearch is moving fast and it might be very likely
+that we missed some features or changes. Feel free to change that.
+
+## Your Pull Request
+
+To make it easy to review and understand your changes, please keep the
+following things in mind before submitting your pull request:
+
+* You compared the existing implemenation with the Java API, did you?
+* Please work on the latest possible state of `olivere/elastic`.
+  Use `release-branch.v2` for targeting Elasticsearch 1.x and
+  `release-branch.v3` for targeting 2.x.
+* Create a branch dedicated to your change.
+* If possible, write a test case which confirms your change.
+* Make sure your changes and your tests work with all recent versions of
+  Elasticsearch. We currently support Elasticsearch 1.7.x in the
+  release-branch.v2 and Elasticsearch 2.x in the release-branch.v3.
+* Test your changes before creating a pull request (`go test ./...`).
+* Don't mix several features or bug fixes in one pull request.
+* Create a meaningful commit message.
+* Explain your change, e.g. provide a link to the issue you are fixing and
+  probably a link to the Elasticsearch documentation and/or source code.
+* Format your source with `go fmt`.
+
+## Additional Resources
+
+* [GitHub documentation](http://help.github.com/)
+* [GitHub pull request documentation](http://help.github.com/send-pull-requests/)