Implement canonical output mode for stream writers

Since there is need for canonicalization of xml documents (for purposes of calculating signatures, and comparing physical equivalence of xml documents), it would be useful if XMLStreamWriter implementations could be configured to optionally produce canonical xml output, as per XML Canonicalization specification. Since such a mode necessarily reduces fully streaming capability of output (attributes have to be ordered in specific way), this should be left as an optional feature, disabled by default.

Due to number of features to go in 3.0 release, this will probably need to be a post-3.0 (3.1, 3.2, 4.0?) feature.