Fix bug that encodes invremaining incorrectly

Closes #66 (closed)

This bug stemmed from changes made to the lox_zkp library to fix a problem where the a public attribute with value 0 would fail. Unfortunately, the changes made to the blockage_migration protocol to account for that change were incomplete. To be precise, the number of invitations remaining, the invremain attribute, was encoded incorrectly (or rather, not at all) at issue time. This still succeeded with a level 3 credential because the value for invremain would be 0 which would cancel out the value when computing the MAC, but when invremain is 1, it must be used to compute the MAC.

This corrects the omission of invremain in the blind issuance.