Streaming of UBI

[santisiri]

• Removed the mintAccrued function and directly displays on balanceOf the accrued value for a verified submission.
• Removed the Snapshot functionality from UBI.sol
• Implemented test cases for verifying token balances that are being accrued in real time.
• Implemented optimizations referred on https://docs.google.com/document/d/1PfdWowMDtSxaXyqvtST_i3TkolM7V7Qont1gccHNhPI/edit#

This version of UBI only requires the user to call startAccruing and its token balance will update in real time. If the user is no longer in ProofOfHumanity, the accrued value will be lost and only the accrued tokens that where minted during an interaction (transfer or burn) while he was a verified submission will be kept on his balances.