Export initialized worker in openpgp.worker

Currently if you initialize a worker with openpgp.initWorker you can't directly access the newly created AsyncProxy object. Access to methods like AsyncProxy.prototype.decryptKeyPacket therefore requires a second worker instantiation.

With this PR:
- openpgp.initWorker exports the created AsyncProxy in the new attribute openpgp.worker
- openpgp.initWorker returns a Boolean which indicates successful Worker initialization
- removes the warning "You need to set the worker path", see also https://github.com/openpgpjs/openpgpjs/issues/269
- a helper method is added to get a Keyid object from a hex keyid