Improved logging

[thelinuxlich]

• options.logLevel can be a string, where its value must be 'debug', 'verbose', 'info', 'warn', or 'error'
• options.log can be an object with {debug, verbose, info, warn, error} methods
• When options.log is a function, its second argument will be the log level of the given message
• When options.log is undefined, the console methods are used by default
• When options.log is falsy (or options.silent is truthy), log messages are silenced
• Some log messages have been tweaked, and log levels attached
• The pool master now emits an 'error' event with the associated Error object

[thelinuxlich]

@aleclarson are you willing to rebase your work on our fork?