Improve Windows support

* Symlink `/etc/nginx/sites-enabled/otm` directly rather than via `configs`
* Use correct syntax for rsync excluded folders
* On subsequent `vagrant up`s:
    * avoid clobbering nginx configuration by ignoring `config` directory when rsyncing `.`
    * use true (non-rsync'd) shared folder (and copy in `local_settings.py`)
Merge pull request #10 from OpenTreeMap/support-windows

Improve Windows support