Added ability to import b2dJson world contents into an existing b2World

This pull request adds the following methods to b2dJson:

```c++
void readIntoWorldFromValue(b2World *world, Json::Value worldValue);
void readIntoWorldFromString(b2World *world, std::string str, std::string& errorMsg);
void readIntoWorldFromFile(b2World *world, const char* filename, std::string& errorMsg);
```
These methods allow a json files contents (excluding the world properties) to be loaded into an existing b2World.