Add new DBI sub, foreign_hash().

Basically a row_hash() function that uses a foreign key column instead of primary key. Example:

code     ticket      type
1234     9876        request
1233     5555        response

my $rec1 = $db->row_hash('1234');
my $rec2 = $db->foreign_hash('ticket','9876');

Each of the above would return the same record hash.

Of course the foreign column has to be unique, or an array will be returned.