Remove indentation and margin from Builder::XmlMarkup calls

[mbrung]

In Sekken::Message and Sekken::Envelope Builder::XmlMarkup is instantiated with margin and indent values, which are used when emitting the XML. Unfortunately some SOAP servers (case-in-point: MS Navision) is pretty finicky about the way requests are formatted, and any extraneous whitespace yields an Invalid SOAP request error.

I understand margins and indentations help emitting a cleaner output (e.g. when using operation#build); should we consider making this configurable somehow?