Create a 2.x branch with php7-only code ready to be refactored.

[bircher]

The 1.0.0 release is stable and caters to a wide variety of php versions (from 5.4 onwards).
To refactor the code with php7 constructs we introduce a small API change, while at it we can better define what the API is by declaring classes as internal and/or final.

The \PhpMerge\PhpMergeInterface::merge signature changes only adding the string type hint, so using it in code as described in the readme will continue to work.