Add V_TEXT support to allow passing text to sensors

[tsathishkumar]

requesting/passing along V_TEXT updates that happen on the server.

Use-case: My smart door lock uses this to get the latest password from the controller. I can then open the door with an SMS by checking if the SMS content matches the password set in the controller.

[flatsiedatsie]

I was wondering: doesn't the Mozilla Gateway already support strings? And manipulating them? I'll try adding a string type after the dust type.

[flatsiedatsie]

I tried to add them and created two pull requests for this. I haven't tested that code, I just tried to write something that was in line with the other items.