Ensure compatibility with Nokogiri

Nokogiri v1.12 introduced a breaking change related to namespace
inheritance of reparented child nodes. This commit using the feature added in
v1.12.4 to opt

- for new versions of Nokogiri that support it, set `Document#namespace_inheritance`
- intermediate versions of Nokogiri will be avoided via gemspec version specification
- old versions of Nokogiri will continue to work

See https://github.com/sparklemotion/nokogiri/issues/2320 for more details.

Fixes #30

fix #26: add xml-exc-c14n Transform when :enveloped option is true.

Add :eveloped_legacy option for backward compatibility.
Add and update the unit tests.

Refactor digest!() method for better extensibility, add GOST-R 34.10/11-2012 algorithms, fix digest node ID reference, cleanup

Refactor digest!() method for better extensibility, add GOST-R 34.10/11-2012 algorithms, fix digest node ID reference, cleanup

add parameter to customize canonicalize algorithm, change x509 issuer content

add references node type on digest
change issuer x509 content to be more standard way

Add wss option for XML only signing

X509 in SecurityTokenReference node

Allow to set up custom xmldsig namespace prefix for Signature node

Add posibility to disable noblanks method in Signer initialization

Remove Ruby 2.0 Support

Update test data

Successful test run completed.