Python25 conversion

Experimentally Fixes issue #38. Note that I did remove two checks for isNaN/isinf. They could be replaced by reimplementing isNaN, isinf...

I did run the tests (on Python 2.7), only 3 failed, but likely with an unrelated error (the error occurred in the tearDown() part of the testcase when trying to `os.remove()` a xlsx file).