Use `logging` module instead of prints in migrater script

I replaced `fail` function printing out to `stderr` with `logger.debug`
(from Python standard library).

Additionally I introduced `info` function that indicates migration
progress.  It simply logs out "Successfully migrated 'TABLE' table",
so that user knows the script is running error-free.
Change default logger name in migrater script

It used to be `"__main__"` because that's what `__name__` defaults to
when invoked directly.  Using string name (`"amy-migrater"`) ensures
that logger has it's own recognizable namespace.
Change `migrater.py` verbosity

For now I stick to hard-coded verbosity of INFO messages (ie. every
message with higher or equal severity to INFO will be shown).
Consult https://docs.python.org/3/library/logging.html#levels for the
list of logging message levels.
Merge pull request #100 from pbanaszkiewicz/morelogging

Use `logging` module instead of prints in migrater script