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
What about empty and comment, as you said, that it should match the base
data dir? As in here
https://github.com/biocore/qiita/blob/master/qiita_core/support_files/config_test.txt#L28