Adds the ability to download a document using the REST API
The document is sent in a base64 encoded format.
The API will return :
{
"filename": "CO1709-0002.pdf",
"content": "JVBERi0xLjMKJ...",
"encoding": "MIME base64 (base64_encode php function, http://php.net/manual/en/function.base64-encode.php)"
}
