Add support for adding Signature Properties to a detached signature

[stan-janssen]

This commit adds support for adding your own Signature Properties when you are using the detached method. This use case is relevant in, for example, the OpenADR specification, which requires a ReplayProtect element to be included in the signature.

I added a parameter signature_properties to the XMLSigner.sign method, and a helper method that builds the required SignatureProperties structure and adds it to the result.

I also added a test case for this feature.

This addresses a real-life use case in a (hopefully) non-intrusive manner.

[kislyuk]

Thanks!