Convert aux module gpp_standalone.rb into a script under tools

This PR converts the gpp_standalone.rb auxiliary module into a standalone script in tools. The reason being Metasploit auxiliary modules always deal with hosts, and gpp_standalone.rb doesn't. Doing it this way also allows faster loading time, and you get to write a rspec for it.

Demo:

```
$ ./cpassword_decrypt.rb AzVJmXh/J9KrU5n0czX1uBPLSUjzFE8j7dOltPD8tLk
[+] The decrypted AES password is: testpassword
```

Rspec:

```
$ rspec spec/tools/cpassword_decrypt_spec.rb 

CPassword
  Class methods
    .decrypt
      should return the decrypted password as 'testpassword'
      should return an empty string due to a bad password

Finished in 1.03 seconds
2 examples, 0 failures
```