v1.7.0

Introduce new PSR-18 based client (#257) @Art4

* Create Psr18Client

* Create interfaces for Client and Api

Move Api instantiation into trait

* Issue Api uses getApi on client

* Add new request methods to ClientInterface

* Add tests for new ClientInterface methods

* Add test for requestGet method

* Test Request created by client

* Move tests for request generation into integration test

* Improve integration tests

* Move clients into own namespace, rename interfaces

* Implement auth with username:pwd or access key

* move Api interface

* add content-type header

* Let AbstractApi consume new Client interface

Move json- and XML-decoding into AbtractApi
Fix and improve all tests

* test exception messages

* Implement impersonate user

* Test correct response data in client

* Add tests for AbstractApi

* Add tests for decoding in AbstractApi

Simplify error messages in JSON decoding

* Test XML decoding on post, put and delete requests

* Add integration tests for POST, PUT and DELETE

* Test file upload with content and file path

* Remove debug code

* Update example.php to use new client interface

* Update README.md

* Create usage docs

* Add docs for user impersonation and curl options, improve language

* Improve AbstractApi, remove obvious docblocks

rename $data to $body
rename $decode to $decodeIfJson

* Rephrase trait description, add @internal

* Update manual installation, link to docs

* Add tests for get api over magic getter

* Update requirements in README.md

* Move usage docs into README.md