Credential Gem: LoginScanner - auxillary/scanner/ftp/anonymous.rb

Refactor 'auxillary/scanner/ftp/anonymous.rb' to support the Metasploit Credential gem

__Note:__ If this module was properly rewritten it would look almost exactly like Tod's example ftp_login.  I chose to instead retrofit the Credential Gem onto the existing structure.  If it is desired, I can instead rewrite it to look like Tod's example but with default tuned for only anonymous login.  It might be better to remove the module altogether.

**Verification**

- [x] check out the metasploit-credential branch by using the instructions in the section 'Check out the electro-release branch' at the link below:
      https://github.com/rapid7/metasploit-framework/wiki/Metasploit-Loginpalooza

- [x] ````msfconsole````
- [x] ````use auxillary/scanner/ftp/anonymous````
- [x] ````set RHOSTS <target_ftp_server>````
- [x] ````run````
     - [ ]  Scan results should be printed
- [x]  ````creds````
- [x]  __VERIFY__ that successful credentials identified in the scan output are found in the results from creds