Add sign

[aljones15]

Adds a sign function to the binary.

Basic Example Command:

cat ../http-signatures-test-suite/test/latest/input/default-test.httpMessage | ./bin/http-signature-header-js-sign --headers "Digest" --private-key ../../../keys/ed25519.private --keyId ./node_modules/mock_keys/ed-mock-pub.json

Passes:

• MUST require key type.
• MUST base64 encode signature (I believe it is)
• MUST produce an error if the key type type is not in the http signature types
• MUST produce an error if the --algorithm is not in the http signature registry
• MUST produce an error if the --algorithm is deprecated
• MUST sign if the --algorithm is not deprecated i.e. hs2019
• MUST not process if --created is in the future
• MUST not process if --expires is in the past