Adding bandwidth and updating readme

[MatthewLJensen]

I've implemented the SMS interface for Bandwidth.

It supports 1 on 1 sms and mms chats. Let me know if there is a way to support group messaging, I just did not see it in the other providers.

There is an optional setting within bandwidth to supply a username and password on callback messages. This authentication is also supported within this interface via the optional 'callback_user_id' and 'callback_password' fields.

This interface does not currently listen for the callbacks that bandwidth supplies that indicate the status of a sent message. Those statuses include 'sending', 'delivered', and 'failed'. So, it is recommended that these are disabled on Bandwidth's side (via a checkbox for each type) until they are supported. Thus a message is 'delivered' when it is received at the carrier.