Cleanup and modularize RXV473 into an rxv module

Normally I'd break these up for a pull request, but I know you're anxious to get these merged in. :) Thanks for reaching out to me on these changes, always glad to help!

Here's the change list:
* Use unix line endings
* Remove unused variables
* Adhere to PEP8 (mostly around line length)
* Simplified a LOT of XML generation
* Added an RX473.off method, to turn off the unit
* Added an RX473.volume_fade method, to gradually fade the volume
  to the specified dB level
* Removed hardcoded IP address in class
* Revised main with simpler and tested example

* Create rxv module
* Auto export RXV473 in new rxv module
* Change rxv473.py back to 0644 and remove hashbang
* Move test code to /test.py