Handle MySQL FK column names idenfier case, fix #62.

The code forgot completely that MySQL column name references in foreign
key definitions have to follow the identifier case rules, this patch fix
that.

To be able to do that, we need to parse the GROUP_CONCAT() result that
lists the FK columns, as there's apparently no arrays in MySQL. The
problem here is that about any character is allowed in column names when
`quoted`, so using a comma here might reveal to be fragile later.