Fix #275: Unset and reset shellslash if on Windows

cmd.exe can't handle single-quoted strings, which shellescape() produces
on all platforms *except* Windows when `shellslash` is not set, when it
will produce double-quoted strings instead. This commit unsets and
resets `shellslash` if needed.

Tested by running GoInstallBinaries after removing binaries on Windows 7
running Vim 7.4.