Add phpstan static analysis tool

Author: Xerkus

composer.json

@@ -26,6 +26,7 @@
         "zendframework/zend-stdlib": "^3.1"
     },
     "require-dev": {
+        "phpstan/phpstan": "dev-master",
         "phpunit/phpunit": "^6.4.1",
         "zendframework/zend-coding-standard": "~1.0.0",
         "zendframework/zend-i18n": "^2.7.4"
@@ -66,6 +67,7 @@
         ],
         "cs-check": "phpcs",
         "cs-fix": "phpcbf",
+        "phpstan": "phpstan analyse -l 7 -c phpstan.neon src/",
         "test": "phpunit --colors=always",
         "test-coverage": "phpunit --colors=always --coverage-clover clover.xml",
         "upload-coverage": "coveralls -v"