Add support for Windows UNC paths specified as --find-links

This pull request allows specifying windows UNC paths to the --find-links option.

At our workplace, we have the need to host our python packages in-house on a network share.

Pip did not allow installing from this share. This patch provides the use case.

    pip install --no-index -f \\unc\as\path package
    pip install --no-index -f file://///unc/as/path package 

Please advise on the quality or approach.

Fixes #1846 
Might be worth it, but I really don't understand how Link objects work. The original code doesn't call ```path_to_url``` at all, so presumably it only expects to get a URL. The new code seems to imply that it can be called with a UNC path (apparently as part of dealing with ```-f \\UNC\path```). I'm not clear why ```-f``` with a normal path wouldn't take this route as well.

Checking for the ```startswith``` method feels like a hack.

I'm OK with leaving this in, but only because I don't want this patch to get hung up on peripheral details. If any of the other devs can comment on what's going on here, that would be great - @dstufft @jezdez @qwcode any comments?