Add support for HTTP Basic Authentication

In @rbates screen-cast on Resque (http://railscasts.com/episodes/271-resque) he shows how to configure ResqueWeb with HTTP Basic Auth.

This pull request adds support for HTTP Basic Auth with ResqueWeb out of the box. To turn it on you just need to set the environment variables: `RESQUE_WEB_HTTP_BASIC_AUTH_USER` and `RESQUE_WEB_HTTP_BASIC_AUTH_PASSWORD`. This makes it dead simple to use for example if your using Resque with Heroku. Just run `heroku config:set RESQUE_WEB_HTTP_BASIC_AUTH_USER=user RESQUE_WEB_HTTP_BASIC_AUTH_PASSWORD=secret` to get ResqueWeb secured with HTTP Basic Auth.