remove bool conversion

as discussed in #16

The `bool` useage is unlikely to be broken since `if some_col_value` equals `if bool(some_col_value)`, but it exposes the raw data from mysql, which is more suitable for a replication lib.
What do you think about just handling the conversion here? The problem is that, the event does not carry the information that the original source column was a TINYINT(1). So you cannot do the conversion in the app just my looking at the event.