Fixed warning with Countries::doCall() method + minor enhancements

Hello,

I fixed a small problem that I found. Every time the `Countries::doCall()` method got called without an argument (e.g. [here](https://github.com/jeroendesloovere/countries-php-api/blob/master/src/Countries.php#L-100)), a warning like the following was generated:

```
Missing argument 1 for JeroenDesloovere\Countries\Countries::doCall(), called in /path/to/repo/src/Countries.php on line 100 and defined in /path/to/repo/src/Countries.php on line 25
```

I also made some minor enhancements. I updated the `example.php` file to use the composer autoloading instead of requiring the class manually.

I added a `.gitignore` file to ignore the `vendor` directory and the backup files that are generated by some editors (files postfixed with `~`).

And finally, I added an installation section in the `README` file that explains how to install the class by using composer.