Support Python distributions without decimal.py

If there's no decimal.py ujson crashes with a segfault because of an unchecked PyImport_ImportModule return value. Since the only reason to import decimal.py is to check whether some Decimal object has been passed in, it makes sense to simply ignore the error and assume no Decimal is present. And because decimal.py is *huge* and therefore not desirable in some embeded configurations, for example, this also improves ujson's overall footprint.