adding basic auth support to rest tab

I noticed the REST tab doesn't support basic auth yet. With this change you can enter `http://user:pwd@localhost:9200/_search` in the REST request panel and it will add the Authorization header to the request.

I should note that this doesn't populate the REST request panel with the URL from the location query parameter. You can still manually change the URL to have the basic auth credentials then it works.

I ran `npm install` and `grunt build` to lint, test, and build it.