Uploader context

Max,

Thanks for the great gem.   I found it impractical to use the session object to pass the additional context information I needed for an uploaded object, so I added a context (text) attribute to the S3Multipart::Upload model itself.  This can be set via a data-context attribute on the file input.   The attribute can be populated with a Javascript object and parsed as JSON on the server.

I also merged in my other PRs and fixed the package.json to fix grunt at 0.3.17 otherwise npm will install grunt 0.4.x and grunt commands will fail due to your dependency on underscore (I think).   I tried to resolve that dependency to move up to lodash, but wasn't able to get that working.

HTH,

-Michael