Connection Pool

Added a very simple connection pool. Users can now specify one or
more Elasticsearch server URLs when creating a Client via NewClient.

Example:

  client, err := NewClient(http.DefaultClient,
    "http://127.0.0.1:9200", "http://127.0.0.2:9201")

The client then starts up and periodically checks for an available
connection. It takes the first connection available and uses it
for subsequent requests. The interval for periodic checks is currently
1 minute.

In case no active Elasticsearch server is found, the client will
return ErrNoClient when creating a new request via NewRequest.
Update README.md