Added support for uploading files to buffer instead of filesystem

A while ago, I encountered a problem uploading images >1MB to a Nodejitsu instance. I assume that cloud hosting providers like Nodejitsu do not always support a large filesystem for node.js instances. Since I needed to upload the file to the server before pushing it to an image hosting service, I modified node-formidable to support uploading files to Buffers instead of to the filesystem.