Symlink support

Damn this was hard.

If I knew it would have been this hard I'd probably wouldn't have started it.
Wrote and threw away so much code X_x

Suggestions for tests?
I guess just add some symlinks in the integration tests?

This will obviously cause Windows nodes running without admin privileges to never get in sync if there are some symlinks. Perhaps this should be optional?

Integration tests still running:
http://build.syncthing.net/job/syncthing-integration/79/