WIP: Download files with nginx

@antgonza helped with this one.  Remaining to be done:

- [ ] Get nginx into Travis CI
- [ ] Add download links in the GUI where appropriate
- [ ] Fix whatever tests are about to fail when I submit this PR
Actually no -- it needs to be the base_data_dir, which defaults to a path relative to the location of qiita_core/configuration_manager.py, so I'm not sure what to put here -- maybe just a comment that says "CHANGE ME: This should match the qiita base_data_dir from your configuration" or something?