Merge of nullable params back into master repo

[mgrundkoetter]

No description provided.

[bnf]

Thanks for merging our branch, much appreciated!

One note though, the branch relied on a open pull request in gossi/php-code-generator:
phpowermove/php-code-generator#64 – therefore I didn't open a pull request to your repository yet.

I should have added the patch to the composer.json in our nullable-getter-returndeclarations branch – anyway, here is a pull request to do that now:
#2
This fixes the API (re)generation. That change can be removed once gossi/php-code-generator merges the pull request.