Reviewer Rob - Revert rf definitions

This undoes my previous dubious change to include our own ACR definitions.  Also includes some fixes required for the Ralf code you're currently looking at:

 * Adds two new AVP definitions (which we might wish to send back to freeDiameter).
 * Allows client code to call `encode_value` on AVPs that don't have custom encoders to have the same effect as calling `setvalue`.