* recreate PR from #400
* first tests with slate encryption
* simplify slatepack model to contain encryption header in payload, and add de/ser tests
* update tests and confirm slatepack encryption working
* remove recipient list, add version check warning
* initial addition of ov3
* move libwallet address functions into OnionV3Address type
* incorporate OnionV3 type where possible
* factor out manual dalek pubkey manipulations
* corrections due to test failures
* change json-rpc example to use ov3 for payment proof recipient address
