Add default constructors for BoundingBox and Model

[TokyoSU]

(This PR come from #198)
This is not finished, i will add more later :)
PS: you can also check the progress here.

[RobLoach]

I think this is pretty good to merge as is 😉

[TokyoSU]

not really, there is more like window, audio etc.. that need it too

EDIT:
do you think changing bool to void would change something ? like:

bool Init(int width = 800, int height = 450, const std::string& title = "raylib") {
    ::InitWindow(width, height, title.c_str());
    if (!IsWindowReady()) {
        throw RaylibException("Failed to create Window");
    }
    return IsWindowReady();
}

to

void Init(int width = 800, int height = 450, const std::string& title = "raylib") {
    ::InitWindow(width, height, title.c_str());
    if (!IsWindowReady()) {
        throw RaylibException("Failed to create Window");
    }
}

since throw would already take care of error ?

[RobLoach]

That would seem appropriate. Good thinking 👍