Add a Logger class and associated helpers (e.g. log formatter)

Do incredibly basic work to instantiate logger and pass it to all plugins
Add a "source" to all log events.
Rename Logger to LoggerManager, factor out log entry creation function
into a Logger object, multiple of which can be created by the LoggerMangaer.
Multiple Logger objects feed log entries to a central LoggerManager, where
they are emitted.
Replace some of the console.log statements in generator.js with _logger.log statements
Merge pull request #168 from adobe-photoshop/jrb/logger

Add a Logger class and associated helpers (e.g. log formatter)