Support For Document Embed API

ISSUU has added support for the new embed code in it's API. This pull request adds support for those methods. There is one method with a different response format (issuu.document_embed.get_html_code), that maybe you'd like to handle differently, but here's my attempt. All the other methods, issuu.document_embed.add, issuu.document_embed.update, etc follow the same format as those for documents and folders.