Added more default namespace prefix combinations

[cmordue]

These are taken from https://www.w3.org/TR/xmldsig-core2/#sec-Versions

[bjrmatos]

LGTM, but one question. seems like dsig2: 'http://www.w3.org/2010/xmldsig2#' refers to XML Signature 2.0, does this mean (technically) that it is a another algorithm/transform to support? i mean, it seems that xml nodes inside dsig2 signature are somehow different. is there any chance that introducing the dsig2 mapping could cause any incompatibility? (xml signature 2.0 seems to have a compatibility mode so maybe there is no compatibility issues)

[cmordue]

@bjrmatos It does sound like xml signature 2.0 is backwards compatible but I'm not certain there won't be problems. I haven't tried any xml signature 2.0 docs. I'll take it out.

I've submitted this primarily because this would have made it so I didn't run into #84 for the dsig namespace (I've already resolved this issue for me by following #105 here: leandrob/saml20#20).

[cmordue]

closing since doesn't appear to be useful for others