Exports C14nCanonicalization, ExclusiveCanonicalization

[angeljqv]

Closes #331

This allow us to do

const C14nCanonicalization = require('xml-crypto').C14nCanonicalization 
const ExclusiveCanonicalization= require('xml-crypto').ExclusiveCanonicalization

// Use Example 
const DOMParser = require("@xmldom/xmldom").DOMParser
let xml_string = '<root><child>123</child></root>'
let documentElement= (new DOMParser()).parseFromString(xml_string).documentElement

// C14nCanonicalization
console.log( (new C14nCanonicalization()).process(documentElement, {}).toString() )
// ExclusiveCanonicalization
console.log( (new ExclusiveCanonicalization()).process(documentElement, {}).toString() )

This is not a breaking change
Any correction I will try to correct immediately

[cjbarth]

Can you please create a similar PR against master so we can keep things in sync?