Add timeout options to Elasticsearch::Client

Adds an optional hash argument to Elasticsearch::Client, which at the moment can
accept :timeout and :open_timeout keys, corresponding to the keys of the same
name in the RestClient::Request constructor.
Add conservative default timeouts

In correspondence with the default timeout of the content API adapters (4s) set
a default connection timeout of 2s and a default between-reads timeout of 2s.
Merge pull request #83 from alphagov/timeouts

Timeout if elasticsearch is misbehaving