Add lookup to bin parent directory for autoloader.

In my `composer.json` file I have a custom `vendor-dir` defined:

```
"config": {
        "vendor-dir": "Sources/Classes/ThirdParty/"
}
```

This installs vobject in `Sources/Classes/ThirdParty/bin`
with the autoloader being located in `Sources/Classes/ThirdParty`

so when `vobject` is called, it can't find the autoloader based on the current code.  It should also look at the `bin` parent directory as well.