Use Go 1.4 build env

This bumps the Docker image to 1.4rc2 and changes the symlink implementation to use Go native. This means *Windows* builds need to be done with Go 1.4.

Those that might not want to go to 1.4 yet are probably the distribution people (Arch etc), and they won't need to change yet so this feels fairly non-impacting to me - we are probably the only, or at least *main* people doing Windows builds.

With this in place we could merge the versioning stuff to fix symlink handling and also cover Windows by default.

There's a build of this in http://build.syncthing.net/job/syncthing-branch/1385/