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 
This test relies on the suspect use of ```ntpath``` in ```Link``` above. There's something screwy going on here. I suspect ```Link``` takes a URL in its constructor, and you're giving it a path here, which is wrong... (same comment for the next 2 tests you added below)