Use approved PSR-15

Fixes #13

Merge pull request #12 from tuupola/new-psr-15

Support latest PSR-15

Merge pull request #10 from webimpress/feature/http-middleware-0.5

Updated to http-interop/http-middleware 0.5.0

Add missing test for named constructor

Add static constructor

Merge pull request #8 from equip/fix/upgrade-middleware

Upgrade to latest middleware spec

Document nested pipes

Ensure initial middleware is typed correctly

Add basic docs and example to README

Add middleware stack implementation