Improve MySQL encoding errors handling.

When it's not possible to decode a MySQL value in the proper given
encoding, automatically replace the value with nil and be quite verbose
about it by logging an error.
Fix #40 by providing a per-table forced-encoding option.

This patch takes benefits from the recent patch
https://github.com/qitab/qmynd/commit/62fc85a1cfa5b04679b9d17461152e9cd4af107d
so that you will need to freshen your local Qmynd copy if you want to
test from sources.