version bump 0.2.6: formula parsing improvements

- xls2csv takes --formulae flag, which dumps all of the formulas
- parser passes the 'math' stress test.  File is at

    https://www.dropbox.com/s/u6co9kmujo44l22/formula_stress_test.xls

This tests all of the "Math and trigonometry formulas" from Excel for Mac 2011