Added internal redirect support

This pull request adds support for serving files stored in swift (or other blobstore) directly via nginx after your application has authorized the download. Full description of the problem can be found [here](http://kovyrin.net/2010/07/24/nginx-fu-x-accel-redirect-remote/).

The functionality can be disabled via `sslproxy.internal_redirect.enabled` which is `false` by default.

There are maybe better ways to solve this problem but we created the pull request to start the discussion with a working solution.