Add OS X persistence local exploit module

This updates @icewall's persistence module (PR #2195, unmerged) with the following changes:
1. Makes it a local module instead of a post module, so that payload is set up for us
2. Adds the keepalive:true option to the launch agent plist
3. Changes the datastore options a bit for minimal configuration
4. Adds an EXE::Custom datastore option, similar to windows persistence module

Verification:
- [ ] Get a shell on some osx machine, run this module. msfconsole will tell you something like:

```
[*] Dropping backdoor executable...
[+] Backdoor stored to /Users/mannix/Library/.pxVEJoHY/com.system.softwareUpdate
[+] LaunchAgent added: /Users/mannix/Library/LaunchAgents/com.system.softwareUpdate.plist
```
- [ ] log out then back in on the target machine. you should see your session drop and then reopen:

```
[*] 192.168.0.6 - Command shell session 7 closed.  Reason: Died from EOF error.
[*] Command shell session 8 opened (192.168.0.6:8888 -> 192.168.0.6:55599) at 2013-10-12...
```