Added name to export directive

Hi, I came across your repo (via a mailing list) while trying to solve the same problem. It was good to know I wasn't the only one :)

I've made a fork to demonstrate the fix. You'll need to replace:

    #' @export

with

    #' @export Blob

and then `export(Blob)` will appear in the `NAMESPACE` file.

Hope that helps.