Generate SHA256 hashes for releases

[systembell]

The sha256 hashes appear to be missing for the Kibana downloads on elasticsearch.org.

Example:

$ curl https://download.elasticsearch.org/kibana/kibana/kibana-3.1.1.zip.sha1.txt
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Key>kibana/kibana/kibana-3.1.1.zip.sha1.txt</Key><RequestId>F36E48FD84190BD1</RequestId><HostId>bfGZtFE+Hwx5ObRtICWqml/aj3NSOjxid9W7ld547lGGWp3EF1uzPtFsLQdug+he</HostId></Error>%

[zabbal]

Better sha256: sha-1 is completely broken long time ago.

[rashidkpc]

We've added sha1 hashes to the downloads as this is the standard for the other projects