Fix #215: Helper for int and double now work as expected.

Was just following http://legacy.python.org/dev/peps/pep-0008/#function-names

Made an assumption that function name style took priority over global variable name style. Maybe a better way to handle it is:

```python
INT64_CHECKER = Int64ValueChecker()
# ...
INT64_CHECKER.CheckValue(val)
```

?