Fix issue #37: make misaki work on Windows

Sorry for the mistake in the first commit message. The issue number should be #37. This small fix is mainly to wrap .getPath and .getAbsolutePath for the java.io.File class so that the backslashes (\) in the Windows file paths are normalized to forward slashes (/) so that they don't mess up the computation of the output file names in make-output-filename. All tests pass as they did before the changes on both Linux and Windows.