js: add verify and signature parsing

• src/gpgmejs.js:
  • Added verify method
  • Added verification results in decrypt (if signatures are present in the message)
  • Added a base64 option to decrypt

• src/Signature.js: Convenience class for verification results. Used for e.g. converting timestamps to javascript time, quick overall validity checks

• src/Keyring.js: removed debug code

• src/Errors.js add two new Signature errors