Snapshot 14w20b has been released to fix mobs not spawning naturally.
To get snapshots, open your launcher and press the “New Profile” button. Call it “snapshots” and check the box saying “Enable experimental development snapshots” and save. To switch to the normal version, you can select it in the dropdown at the bottom left corner of the launcher. Back up your world first or run the game on in a different folder (See the “new profile” dialog).
Snapshots can corrupt your world, please backup and/or run them in a different folder from your main worlds.
Cross-platform server jar: https://minecraftlucky.com/dowload/file-3405
Cross-platform server windows exe: https://minecraftlucky.com/dowload/file-3406