implemented parsing of digital signature tables (DSIG):
- added DigitalSignature.as, containing data about a single signature
- added DigitalSignatureTableData.as, containing the signatures of the font
- added DigitalSignatureTableParser.as
- added ITableParser.as, implemented by DigitalSignatureTableParser.as
- moved data type parsing functions to DataTypeParser.as
- Table.as now contains the data field, containing the table data