Improve Logging in Javascript SDK

[peterschwarz]

Currently, the Javascript SDK logs any errors, warnings, info and debug messages to console.log. This doesn't give the application developer any control over logging.

We should provide a way to emit these as events, and the application developer can listen for and pass on the events to whatever logging implementation the want.

E.g. An application developer can get internal messages via:

transactionProcessor.on('log:error', (message) => console.log('ERROR: ' + message))
transactionProcessor.on('log:info', (message) => console.log('INFO: ' + message))
// Uncomment for debug logging
// transactionProcessor.on('log:debug', (message) => console.log('DEBUG: ' + message))