Minor fix and tweaks
- Renamed exporters extension property to _extension
- Removed FORMAT_* constants
- Added _format property to exporters
- Added missing extension to XML exporter
- Added EXPORTERS constant
- Added FORMATS constant (based on items available on EXPORTERS)
- Moved module constants to the end of file
- Updated DTB.export() new FORMATS and EXPORTERS constants
- Updated module --format argument to use new FORMATS constant as choices