inconsistent large number rounding

Thanks for building this really cool library! I noticed the following. 

```
# python:
int(float(255120250923013158)) == 255120250923013152
js2py.eval_js("String(255120250923013158)") == 255120250923013152

// JavaScript:
String(255120250923013158) == "255120250923013150"
```
