[EXT] Added basic integration with the Backup gem

See: https://github.com/meskyanichi/backup/

Example:

require 'elasticsearch/extensions/backup'

Model.new(:elasticsearch_backup, 'Elasticsearch') do
  database Elasticsearch do |db|
    db.url     = 'http://localhost:9200'
    db.indices = 'articles,people'
    db.size    = 500
    db.scroll  = '10m'
  end

  store_with Local do |local|
    local.path = '/usr/local/var/backups'
    local.keep = 3
  end

  compress_with Gzip
end