Add callback for client-specific logging at various log levels

[CJLove]

Add an API which would let the client register a logging callback function which would be called when/if the library were to log anything. This would allow integration with the client's own native logging (whatever that might be).

[CJLove]

Clients can currently register a concrete subclass of the ISerfLogListener subclass via the SerfClient::Monitor() API, which has an onLogRecord() method called with logging records from Serf. Need to decide whether this would also be used to return logging records from serf-cpp.