Added diagram id
Changed :id to :diagram_id.  This is more clear.
Initialize collections as hashes.
Forgot to put in writers
Corrected parse_tables and added parse_table_columns
Renamed class in file.
Added test for DiagramParser.
Rename DiaDatabase to DiagramObject.
Added files to database modules.
Added file to database module.  Also added reference parsing methods.
Added test to validate the database object returned from DiagramParser.