Added attachments handling to ImpersonateMessage

This adds the handling of Attachements to the ImpersonateMessage class, after merging https://github.com/henrikbjorn/Stampie/pull/36.

In order not to break anything, I have to be sure that `"henrikbjorn/stampie": "~0.7",` gets the update, which is not the case at the moment. A tag issue perhaps, as discussed on [Stampie](https://github.com/henrikbjorn/Stampie/pull/36#issuecomment-49405064).

AFAIK, this should wait for the new tag, just to be sure.