cast override suggestion

Hi, guys!

Nice project!

I'm just curious, can't we override the type casts so that we don't have to specify `Json::Value::as*()`?

For example, with your current version, we have to specify the type of the values like:
```cpp
Json::Value json;
int var1 = json["IntParam"].asInt();
std::string var2 = json["StrParam"].asString();
```

However, by applying this pull request, we can do something like:
```cpp
Json::Value json;
int var1 = json["IntParam"];
std::string var2 = json["StrParam"];
```

*without* specifying the type, and leave it to the compiler to figure it out from the context.

You may have discussed this prior to transferring to GitHub, but I'd really like to know why you're adapting explicit type specifications without using the features of C++.

Thanks in advance :)