Added support for finding foreign keys on Hsql databases

I wanted to drop foreign keys on a Hsql database. This seemed to be not possible, so I've added a FindForeignKeyConstraintGenerator for Hsql.
While writing this generator which performs a 'select <column> as <label>' query, I noticed that the column labels got ignored and instead the original column name was shown.

After some research I came across this stackoverflow question: http://stackoverflow.com/questions/12709814/

Using getColumnLabel instead of getColumnName fixes the problem and should maintain compatibility.

JavaDoc: http://docs.oracle.com/javase/6/docs/api/java/sql/ResultSetMetaData.html#getColumnLabel%28int%29