Add cache support

[dkarlovi]

Much like Github has.

https://github.com/KnpLabs/php-github-api#cache-usage

[GrahamCampbell]

We already have cache support. It works in the same way.

[GrahamCampbell]

Well, almost the same way. We don't expose it on the client class itself. You have to pass an http client builder into the constructor of the client, having added the cache plugin to it.

[GrahamCampbell]

$builder = new Gitlab\HttpClient\Builder();
$builder->addCache($cache);
$client = new Gitlab\Client($builder);

[dkarlovi]

Ah, should have checked myself, sorry for the churn and thanks for the reply!