Fix several quoting issues, especially on Windows

[jdelkins]

Most important issue: navigating to C:\Users\joel was translated by the quoting code to \\Users\\joel, which, looking like a UNC path, would fail. Solution is to use Vim's built in fnameescape function to handle path quoting rather than a ruby function with gsubs.

Another issue is that in some cases on Windows, readable? may succeed but the directory may not actually be readable, apparently due to how the permission system works on that OS. This was worked around with a rescue block.

Finally, there appears to be some issues with navigating to directories with single quotes in the directory name. this seems to fix those.

I tested these patches on Linux and Windows 7, in both cases with Ruby 1.8.7. I did not include the rebuilt plugin/* files in the patches for brevity.

[sjbach]

Thanks! This is pretty thankless stuff, so it was really great of you to do it. I did need to make a couple compatibility fixes. Specifically:

• fnameescape() doesn't exist in Vim 7.0/7.1
• Each invocation of :ruby is in its own namespace for Ruby 1.9+

[jdelkins]

My pleasure. I tested your fixes and everything works perfectly on my end. Thanks for a great product.