Adds osx autologin password post module.

Recovers the plaintext password of a user with Autologin enabled. The session must be root.

I am unsure of some things:
- Do I need to tag this module some how as requiring root?
- not sure I am using report_auth_info correctly

##### Verification
- [ ] on your target machine, enable Autologin for some user account (System Preferences -> Users & Groups -> Login Options -> Set Automatic Login to some user account)
- [ ] Run this post module against a root osx session on the box. Verify that the autologin user's password is decoded & printed correctly
- [ ] Run this post module against a normal user session on the box. Verify that it fails with a helpful error.
- [ ] Disable the Automatic Login account in System Preferences
- [ ] Run this post module against a root osx session on the box. Verify that it fails with a helpful error