Add user friendly simple api.

```cpp
picojson::value v;
const char *s = "{\"a\": [1, 2, {\"b\": {\"c\": 3}}]}";
picojson::parse(v, s, s + strlen(s));

try
{
  std::cout << v["a"][0].as<double>() << std::endl;
  std::cout << v["a"][2]["__missing__"].as<double>() << std::endl;
}
catch (picojson::type_mismatch& e)
{
  std::cout << e.what() << std::endl;
}
```

output
```bash
1
type mismatch! call is<type>() before get<type>()
```