Make sure configure returns $this

[Nyholm]

We need to make sure all our configure() calls returns $this.

This is blocked by #532

[bobeagan]

I believe this should be good, the only configure() that doesn't is in the AbstractApi https://github.com/KnpLabs/php-github-api/blob/master/lib/Github/Api/AbstractApi.php#L37

[Nyholm]

Thank you!