Simplify ways that hosts are specified when a client is instantiated

[estolfo]

Right now, the Elasticsearch::Client.new can accept the keys:

• host
• hosts
• url
• urls

And the values can be:

• Array
• Hash
• String

This should be consolidated into one key and one type. For example, just hosts and a list of String or URI objects:

client = Elasticsearch::Client.new(hosts: [ 'http://localhost:9200' ])