Windows fixes

These are the changes required to allow running ``./bin/phpspec run`` on Windows without failure.

The first commit is only dealing about path expectations to use the proper directory separator as the implementation of the code is using it.
The second commit fixes the handling of line endings in the StringEngine:

- returning consistent line endings instead of putting a ``\n`` in one place and using ``PHP_EOL`` for other lines
- exploding the unified diff properly as the Diff library always uses Unix-style line endings
- fixing the expectation to convert the line endings on Windows

I still see an issue in the StringEngine on Windows: it explodes the input using ``PHP_EOL``. This works fine when it uses PHP_EOL but would produce a weird result when the input uses Unix line endings and you are on Windows (which is likely to happen regularly as lots of code is using Unix line endings in strings).