Message attachments

Added support for (multiple) attachments for messages.

Attachments can be added via api or through mail (via mailgun).

Using [CarrierWave](https://github.com/carrierwaveuploader/carrierwave) and fog gem for file handling and uploading to amazon s3 (carrierwave using local file storage in development environment).

To activate uploading to amazon in production amazon web services credentials are required, these can be created using [IAM](http://aws.amazon.com/iam/) over at amazon. Also a s3 bucket for storing the files are required.

Also added simple links to assets in message view (someone with better Warhol eyes may want to update the design).