Add PHPStan and bump to max level (#139)

* composer: put require- together

* add phpstan

* add phpstan to travis

* phpstan fixes

* phpstan fixes

* phpstan fixes

* drop null defaults since every property is null by default

* DescriptionFactory: fix parse nullable type

* travis: add phpstan max

* travis: fix phpstan config

phpstan.neon

@@ -0,0 +1,10 @@
+parameters:
+    ignoreErrors:
+        # false positive
+        - '#Call to an undefined method object::getDocComment\(\)#'
+        - '#Cannot call method render\(\) on phpDocumentor\\Reflection\\DocBlock\\Description\|string#'
+        - '#Calling method create\(\) on possibly null value of type phpDocumentor\\Reflection\\DocBlock\\DescriptionFactory\|null#'
+        - '#Calling method resolve\(\) on possibly null value of type phpDocumentor\\Reflection\\(TypeResolver|FqsenResolver)\|null#'
+
+        # nested parents
+        - '#Calling method render\(\) on possibly null value of type phpDocumentor\\Reflection\\DocBlock\\Description\|string\|null#'