cs-completer: Handle whitespaces in path to solution

Fix for Valloric/YouCompleteMe/issues/1119: put path in quotes to prevent Omnisharp from interpreting a path containing spaces as multiple arguments. I don't have much access to Windows-machines right now, so it took me a while to test it on that platform.
Consider moving the condition into a function. Something like `IsServerRunning` and then you can do `if not IsServerRunning()`. You get the idea.