Fixed support for distributions without decimal.py

ujson crashed Python if decimal.py wasn't
available. This fix makes ujson ignore the problem
and instead assume no Decimal object will be
passed in to ujson.dumps().
Merge pull request #97 from romuloceccon/missing_decimal_fix

Support Python distributions without decimal.py
Fix minGW sprintf_s build error
On windows, We should replace snprintf on msvc only.
Merge pull request #103 from easonlin/master

Fix minGW sprintf_s build error