Fix #80: make gitv work with shellslash and Windows git

Fix error using :Gitv --all on git version 1.8.4.msysgit.0 (from Github for Windows). This works on the older git version 1.7.9 (from cygwin) probably because cygwin git actually handles the quotes like unix and Github for Windows handles them like Windows.

With shellslash enabled, shellescape uses single quotes and the Windows command parser doesn't strip the quotes properly.

Solution: Strip surrounding quotes. We're already doing this for empty arguments, now we do it for all arguments. I don't understand the case where we need to escape the arguments (despite that sounding like the right thing to do), so I'm not sure if this breaks something.

Sorry, this PR is on top of #81. Also, I'm not sure if you were asking for comments in code or in the PR? Let me know and I can fix it up.