Windows: Fix tests & building the mkdocs documentation on Windows

This pull request includes everything in #96.

Notable changes beyond #96:
* Include `ez_setup.py` for easier installation on systems without `setuptools` (such as AppVeyor CI), [as recommended by the `setuptools` docs](https://pythonhosted.org/setuptools/setuptools.html#using-setuptools-without-bundling-it)
* Changed one test to use `tempfile.mkdtemp()` instead of `tempfile.NamedTemporaryFile()` because Windows doesn't like to read open files
* Changed nav.FileContext.make_absolute() to use posixpath instead of os.path because it's only used to compare a filename against mkdocs.yml paths

This weekend I learned it's pretty easy to set up Windows-based CI for this:

1. Go to http://www.appveyor.com/ and click `SIGN IN` in the upper-right corner
2. Connect to GitHub
3. Make a new build, select the `mkdocs` repo
4. [Optional] In Settings &rarr; General, build all branches.
5. In Settings &rarr; Build, select `SCRIPT` and `CMD`, then enter `python setup.py install` and `c:\python27\scripts\mkdocs build`
6. In Settings &rarr; Test, select `SCRIPT` and `CMD`, then enter `python setup.py test`
7. In Settings &rarr; Artifacts, add `dist/*.egg` and the `site` folder (because why not).
8. In Settings &rarr; Badges, copy the Markdown snippet into README.md and tweak as desired.

...Looks like [steps 4- can also be set up with a YAML file](http://www.appveyor.com/docs/appveyor-yml).  Let me know if you'd like me to write/test/push one for you.

I hope I've done my homework well enough.  Please let me know if you still have any reservations about accepting this pull request.  I'd really like to start using mkdocs at work.
I don't think it's Windows-specific: you'll probably see the same failure in Linux if you apply the changes from `setup.py`, and the bottom of `mkdocs/mkdocs.py`, and add `ez_setup.py`.  I'll see about spinning up a Linux VM later today to demonstrate.