Add ability to attach files to events

[newsch]

It would be useful to attach photos/flyers to events, and potentially store them in a separate endpoint at /files/

A couple things that might be helpful regarding file uploading:

• It looks like files are accessed with request.files in Flask requests, but I'm not sure what the difference is between sending files, json data, and non-json put data and how flask handles it http://flask.pocoo.org/docs/0.11/patterns/fileuploads/
• Multiple files with flask: https://stackoverflow.com/questions/11817182/uploading-multiple-files-with-flask
• The field to add to document_models.py: http://docs.mongoengine.org/apireference.html#mongoengine.fields.FileField
• Amazon S3 Buckets