allow to pass extra HTTPRequest kwargs (e.g. request_timeout).

tornado.httpclient.HTTPRequest class has default 20s for request_timeout. It's too short sometimes for my use case. I added an attribute "httprequest_kwargs" to ESConnection class, it will be passed to HTTPRequest class when making actual requests. That way, users can fine-tune some parameters for making HTTP requests if they want.

Cheers,

Chunlei