Multiple styling fixes ✨

• Fix readme to fit PSR-2 (4 spaces indentation)
• Add ext-json in the composer.json as it is required for json_decode.
• Fix some PHPDocs that were missing stuff, had typos or were missing.
• Remove unused imports
• Add the $visitors class variable to IntercomClient. It was missing, but due to PHP's magic, it was created on the fly by the constructor.

All credits go to PHPStorm ©️

My next step is to check for broken links in the PHPDoc blocks and fix PSR2 in the tests to be able to add linting to the build 😄