version bump 0.5.0: changes to cell interface

Cell object stores the formatted text in the `.w` field
- Raw format and types are preserved in the `.t`, `.v` fields
- Accessors have been updated to use the field

updates for older OOXML:
- ECMA-376 makes reference to old sstItem shared string item
- [MS-XLSX] xmlns for workbook

version bump 0.4.3: shared strings and formatting

- XLSB basic support for number formats
- removed extraneous Strings table element
- formatting xml tags (rich_stress_test)
- updating test_files to 20130130