Support for Basic Authentication

Hi there,

I'm loving your ES plugin for Scout :)  However, in our case, it would be great if it supported Basic Auth since our ES instances sit behind a proxy which provides authentication.

I've made a slight modification to your script which allows for option(:basic_username) and option(:basic_password), and adds it to the Net::HTTP call.

Honestly, I'm not too familiar with how the Scout plugin development process works, but I'm hoping this pull request is something you can merge and update.

Cheers!
Adam