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 add it to the core FieldParser class.