Add a reasonable to_s method to models

in a workspace with at least one each of: Password, NTLMHash, and SSHKey
 - [x] msfconsole
 - [x] `irb`
 - [x] `Metasploit::Credential::Private.all.map(&:to_s)`
 - [x] `Metasploit::Credential::Public.all.map(&:to_s)`
 - [x] `Metasploit::Credential::Realm.all.map(&:to_s)`
 - [x] **Verify** it doesn't blow up and that output is reasonable

## Post-merge steps
- [x] Edit `lib/metasploit/credential/version.rb` and increment the PATCH constant
- [x] Verify that 'rake spec' passes all specs
- [ ] Commit and push
- [ ] Create the appropriate tag
- [ ] Push the tag