Support environment variable for patch output path

[siemova]

Resolves #12 by adding support for environment variable VENDOR_PATCHES_OUTPUT_PATH. Its value may be absolute, but must ultimately be relative to the repository. If so, PatchFileFactory::createPatchFilePath() will use it; otherwise, it will fall back to the current default path: "patches".

Incidentally fixes some directory separators under Windows, and a few typos.