Replace direct_upload_form_for with form_tag

We want to support multiple users uploading to their own S3 buckets.
This doesn't appear to be possible/easy with CarrierWave so we're
planning to replace it with uses of the AWS SDK instead. This is the
first step in that direction.
Remove carrierwave & carrierwave_direct.

We're now using the AWS SDK gem to take care of the uploading to S3.
Improvements to the README.

* Added explicit instructions on how to create a user with the correct
permissions.

* Move the setting of the default region to a more appropriate position.
Explicitly set AWS credentials.

We want to be able to change the AWS credentials on a per user basis.
This is the first step in that direction.
Multiple users.

* Introduce a User model to store user-specific AWS credentials, etc.

* Use HTTP Basic authentication to distinguish between users.

* For the moment we're passing the current user into methods on the
Video model which is a bit ugly, but we hope to improve this later.