update after cleanups

[dbu]

• HttpClient instead of HttpPsrClient and HttpMethodsClient
• Utilities for HttpMethodsClient
• Client instead of Adapter
• httplug composer package name

[sagikazarmark]

I would say we have to reread and rewrite the whole documentation, as it became fully outdated.

@dbu updated your comment, made it a todo list.

[dbu]

fixed in #10