Make basic-auth plugin working

There is a basic authentication plugin and my [modification](https://github.com/karussell/elasticsearch-http-basic) makes it possible to be used from an ajax query (enables OPTIONS queries).

Now we need basic authentication for elasticsearch-head in order to work around the limitation of adding custom header parameters to the elasticsearch response (which could be used to challenge the browser to let the user enter the password)

To use this you have to add auth_login=yourlogin&auth_password=yourpassword to the URL of elasticsearch-head. The elasticsearch configuration should look like:

> http.basic.user: yourlogin
> http.basic.password: yourpassword
> http.cors.allow-headers: "X-Requested-With, Content-Type, Content-Length, Authorization"