Async Upload with Celery

[bbengfort]

Right now the file upload process is synchronous and the following computations are performed:

1. Read the file for the hash value
2. Read the file for length, no. dimensions
3. Insert record/collision into database
4. Store on S3 if success.

Especially since we're moving to dropzone.js (#3) we might as well make this computation chain asynchronous using Celery tasks. This will allow us to do better detection and also give variable status reports for different file uploads.

This might help some of the problems/questions cited in #7.