Formulas, dates, numbers support

0.6.0a (19.07.2005)
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* blanks, numbers, currency (use number formats!) and dates (use number formats!)
* pyExcelerator uses 1.5-2 times less memory (thanks to __brains__ and __slots__)
* fixes for Big Endian CPUs
* some refactorings
* new examples: blanks.py, merged1.py, dates.py, numbers.py, num_formats.py, formulas.py
* most important change: formulas support