graphs.get_ids_by_host and get_or_create_for_host modify

1. I want to assign a unique name to "screen name".
2. I want to separate each function in the screen. Therefore, I want to permitted the introduction of a filtering function.

ex. put on the screen a graph of "host-one", "host-two" and that includes CPU word.

```ruby
#!/usr/bin/env ruby

    zbx = ZabbixApi.connect(
      :url => 'http://www.example.com/zabbix/api_jsonrpc.php',
      :user => 'foo',
      :password => 'bar'
    )

    host = ["host-one", "host-two"]

    graphids = []
    host.each do |h|
      graphids << zbx.graphs.get_ids_by_host(:host => h, :filter => "CPU")
    end

    graphids = graphids.flatten

    zbx.screens.get_or_create_for_host(
      :screen_name => "test screen",
      :hsize => 2,
      :valing => 3,
      :halign => 3,
      :height  => 100,
      :width   => 600,
      :graphids => graphids
    )
```

Please merge this commit.
thank you.