⬆️ Allow php 8 (#6)

Allow guzzle 6 (#4)

* Set minimum php version to 7.2 and minimum Guzzle version to 7.0.1

* Allow guzzle 6

Add support for Guzzle 7 (#3)

* Set minimum php version to 7.2 and minimum Guzzle version to 7.0.1
* Make it non-breaking for people who use Guzzle 6
* Allow Guzzle 7.x versions instead of 7.0.x
* Allow Guzzle 7.x versions instead of 7.0.x

Merge pull request #1 from DanielGSoftware/master

Add support for composer 2.0

Added LetterQueue, multiple metadata additions for a letter and prepa…

…ration for unittests

Added example for sending a letter

Added README with examples

added psr-4 support

added psr-4 support

Merge branch 'master' of github.com:postbode/postbode-api