Revert "Merge pull request #1078 from syncthing/go1.4"

This reverts commit b658afd85773284c71a12a38d7f1eef4deda7b58, reversing
changes made to 591c5dabf4cfb892a9d26aa8d1a10506e998373b.
Scrap IsSymlink for native support on Go 1.4

Obviously needs Go 1.4 to go back in.

I am still open to doing fix-up's on rescan interval on Windows, which
would still allow getting rid of all the Windows code.

Frankly, we could just defer creations of links (like we defer deletions of files)
in hopes that the target gets created, and if it doesn't, well tough luck, you'll
get a file symlink.

To be honest, nobody would even notice this 'issue' as I am sure nobody on
Windows uses symlinks.

But at the same time, this ugly code is hidden away in some creppy file in
it's own module far far away, and the interface that it exports is fine'ish,
so I wouldn't mind keeping it as it is.
Use Go 1.4 build environment (currently 1.4rc2)
Merge remote-tracking branch 'origin/pr/1082'

* origin/pr/1082:
  Scrap IsSymlink for native support on Go 1.4
Run filetype and symlink integration tests with versioning (ref #1071)
Handle broken symlinks, Simple versioner (fixes #1071)
Handle symlinks, Staggered versioner (fixes #1071)