Enable initialising Logger with no arguments

[marcusmunch]

Logger$generate_filename() has been changed to an active binding Logger$log_filename

With these changes, the Logger will not require db_tablestring and ts to be set until Logger$log_filename is called, and only when attempting to write an actual file (closes #17).

Instead of taking arguments log_table_id and/or log_path (or neither), update_snapshot now may take an already initialized Logger object with the logger argument, allowing for better control of console output (closes #16).