Write a MXit transport

Basic idea is to use the Mobi portal API:

1. Identify using the given [HTTP Headers](http://dev.mxit.com/docs/mobi-portal-api#headers)
2. Use the `X-MXit-Contact` name for tagging inbound messages
3. Surface replies as HTML that's valid for MXit
4. For responses that look like multiple choice menus make the menu items clickable.