Add support for signing and signature validation #68

The root bom and individual components can be signed.

This enhancement request is to:

• Add support for signing the bom element and individual component elements
• Add verification of all signatures in the bom (the root bom and all components)
• Support XML Signature (via JSR 105) and JSON Signature Format(webpki.org) for inline signing cases
• Support document signatures using JSON Web Signature (jose4j?)