Added support for currency field parsing

Initial use of library threw error when encountering a currency field.  Documentation suggested adding custom field types by subclassing FieldParser.  However currency isn't really a custom field, it's been a supported field for a while (http://msdn.microsoft.com/en-us/library/aa975386(v=vs.71).aspx)  Therefore it would probably be a better solution to modify core FieldParser class.
Updated currency field (Y) to word in Python 2.7 and to return Decimal.
Added Y field type to docs and updated number of field types in readme.
Added tests for currency field (Y).
Updated changelog.
Updated changelog.