Cleaned up -- imports, export code & packages.

In this release, we did the following:
1. Split Exporter into Exporter and Format;
2. With 1, the API comes out cleaner;
3. Renamed exporters package as export;
4. Combined xml & text modules into formats;
5. Renamed base.Part as base.OrderedPart;
6. Added exports module to export package;
7. Renamed XmlExporter as XmlFormat;
8. Renamed TextExporter as TextFormat;
9. 7 & 8 extend base.Format;
10. Modified OrderedPart.export to return list;
11. Added Format.__add_part();
12. Moved package imports to __init__.py files;
13. Added __main__.py to bom_costing and tests;
14. Added run module to tests package;
15. bom_costing_test renamed bom_cost; added run();
16. Edited README to reflect these changes.