WIP Reimplement Payment Protocol

This reverts the payment protocol disabling commit. As for the other commit, I haven't had time to test this directly in copay yet, but it should work (as far as the features it uses, the tests pass in bitcore).

edit: Also, bitpay/bitcore#498 needs to be pulled and released before this will work.

cc @matiu @maraoz @ryanxcharles @gasteve