Precision for currency values in xls files

[redstreet]

From #70

1. "Cash Withdrawal" entries should have 2 decimal places even for whole numbers but are currently missing. This affects automatic numerical display inference in Fava

Temporarily fixed. Still needs a full solution. The problem is, petl uses xlwt/xlrd to read excel, which use excel datatypes, which we don't want, as we want just the string, which we can then pass on to decimal.Decimal. Not sure exactly how decimals are stored in excel

[redstreet]

stackoverflow xlrd API ref