Add module with Windows symlink functions

Add module "ghar_win", containing Windows implementations of the
symlink-related functions of Python's "os" module.
Add "lstat" replacement to "ghar_win"

The ghar main module uses "os.lstat()" to figure out whether a directory
entry is a directory, file or symlink. Implement a ghar-specific
substitute for "lstat" that can detect symlinks.
Clean up code "ghar_win.py"
Fix handling of symlinked directories on Windows

Add various tweaks to ensure proper handling of symlinked directories on
Windows.
Move 'symlinked folder unlink' fix into 'ghar_win'

Let the "ghar_win" module monkeypatch the "os.unlink" function, so that
"ghar" can just use its original code for removing symlinked folders;
the new "os.unlink" version installed by "ghar_win" can then decide to
call the right function on Windows.
Add note about Windows support to README

Let users know that ghar under Windows requires Administrator privileges.
Clean up comments and docstrings
Merge pull request #24 from onnodb/ghar-windows

Ghar for Windows