Remove /SMS/Messages functionality

Fixes #64. This pull routes all
```ruby
@client.account.sms.messages
```
calls to be the same thing as 
```ruby
@client.account.messages
```

It routes `sms.messages` to the `/Messages` Twilio endpoint. In essence, users of this helper library can no longer reach `/SMS/Messages`. This is okay, since `/Messages` provides the exact same functionality. This solution was chosen over fixing the name collision because it keeps the library's code much simpler.