Make it more convenient for Mac users, so they only have to set the path to the app

This just makes it more simple for Mac users, so they can set ```ARD_HOME``` using:

```
ARD_HOME = /Applications/Arduino.app
```

Instead of:
```
ARD_HOME = /Applications/Arduino.app/Contents/Resources/Java
```

This is also how the path looks is in Windows and Linux as well.