Getting unsigned of value with negative number should throw a ptree_bad_data

As of now ptree will not complain about a negative value and will happily cast it to unsigned type. If that's expected then I think it's highly unintuitive.

```cpp
boost::property_tree::ptree pt;
pt.put("foo", "-1");
std::cout << pt.get("foo") << std::endl; // 4294967295
```