[Proposal] Return success/failure code after specs running finished

Hi, @dmeehan1968.
I'd like to propose returning of the result code after specs running finished.
It might be useful when you're using CI or run your builds from command line for another reasons.
So, `main` function will look like this:

```cpp
int main(int argc, const char * argv[])
{
    ZingBDD::StreamReporter reporter(std::cout);
    return ZingBDD::Runner::getInstance().run(reporter);
}
```

instead of
```cpp
int main(int argc, const char * argv[])
{
    ZingBDD::StreamReporter reporter(std::cout);
    ZingBDD::Runner::getInstance().run(reporter);
    return 0;
}
```

In current implementation the same goal might be achieved by returning this code manually:
```cpp
int main(int argc, const char * argv[])
{
    // ...
    return reporter.failed() != 0;
}
```

But, IMHO, returning result code directly from `run` method looks more natural.

What do you think about this feature?