Added async stream for middlewares and removed combine support

What's Changed

• Feature/ig/async stream middleware by @igashev in #3
• Async stream by @igashev in #4

Full Changelog: 1.3.0...1.4.0

iOS supported version moved to 13 and added async await support

• iOS supported version moved to 13
• Added async await support

Lowered iOS version to 11

In order to make it possible to be imported conditionally into projects.

Tests, Documentation, Readme

This release includes documentation for every functionality. Over 95% code coverage. Finally a readme that would help out when beginning with NetworkRequester.

Introducing Combine into the framework

This release introduces Combine and uses only its features.