Integrate proof of concept gulpfile into Django project

 - Changes the Django `static` and `media` directories to be inside `/var/www/nyc-trees/`
 - Adds the `static` template filter for converting static file to use their versioned path
 - Fixes a few issues with the proof of concept `gulpfile`
Works with the most recent set of changes in https://github.com/maurizi/nyc-trees/commit/ecfb72afe3984aeb37643d223a37c7e93aba4247.