Added Elasticsearch integration

As a complement to rubygems/rubygems.org#455 (“Improved search feature”), this pull request adds integration for setting up Elasticsearch node(s) as the application search service backend.

* The "elasticsearch" [cookbook](https://github.com/elasticsearch/cookbook-elasticsearch/) has been added to the Cheffile

* A Vagrant VM named `search` has been added

* A `search` role has been added

* Node configurations (*.json) for Vagrant and EC2 have been added

* The Capistrano tasks have been updated to reflect the changes

* A Chef template for Tire initializer has been added

To deploy in Vagrant, you need to:

    vagrant up search
    time time cap chef:search

There's full stack running in Amazon EC here:

**<http://54.235.152.92:3000/search?query=depends%3Arack>**

It has been deployed via the Chef and Capistrano integration. The application, database and Elasticsearch servers all run on the `t1.micro` instance, with a limited set of gems preloaded.

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I have encoutered numerous issues when deploying the full stack in Vagrant and Amazon, for which I have submitted the rubygems/rubygems-aws#123 issue.