Remove PhpFileSyncer and add rsync support for Windows

[TravisCarden]

Fixes #175

This eliminates the need for a whole separate, PHP-based file syncer by refactoring the rsync-based one to support Windows.

BC Breaks:

Everywhere:

• \PhpTuf\ComposerStager\API\FileSyncer\Service\PhpFileSyncerInterface has been removed.
• \PhpTuf\ComposerStager\API\FileSyncer\Service\RsyncFileSyncerInterface has been removed.

Windows only:

• The active and staging directories cannot be on separate volumes.
• Due to the previous point, the temp directory may cause problems and is no longer tested.
• The active and staging directories can no longer be nested one inside the other.
• All paths now use forward slashes.

https://github.com/php-tuf/composer-stager/wiki/Limitations-and-known-issues has been updated accordingly.