update go.mod to reflect new versioning scheme

v0.7.1

update documentation

internal refactor

• modularize Header generation with HeaderBuilder interface
• set SHA256 as default HMAC as SHA1 is obsolete
• update documentation

Initial release

Initial Release of SOAP client supporting the following WS-Security x.509 features by default

• Include a wsse:SecurityTokenReference with the signature public key in form of a wsu:BinarySecurityToken
• Automatically add a wsu:Timestamp with validity 10 seconds
• Automatically generate a wsu:Id and use this #ID as URI to reference the respective signed element(s)
• Sign Timestamp + Body elements of the SOAP message