Finalize tutorial

[dbu]

the tutorial added in #10 started the outline for a tutorial. work this out and solve the TODOs

we promise information for library developers in the intro, but never come to that point.

[Nyholm]

This issue is still relevant.

This page needs some love: http://docs.php-http.org/en/latest/httplug/tutorial.html

But it should just be for application developers, not a tutorial for every one like it started as.