BooleanColumn, DatabaseDataSource and new edge renderer added

-BooleanColumn now accepts integer as input. Everything greater than
zero is true. The rest is mapped to false.
- DatabaseDataSource refactored. Processing of a ResultSet is now done
in a new class (DatabaseResultSetProcessor).
- DatabaseResultSetProcessor creates or updates an existing prefuse
table with the data found in a ResultSet.
- MixedGraphEdgeRenderer extends EdgeRenderer with the ability to show
two straight edges between one pair of nodes.