add transferrable HPOOL tokens to pool contract

[chuck-h]

This "tokenizes" deferred payouts in the pool contract. Could be part of the Rainbow project.

    ---begin pool token tests---
    reset pool
    reset accounts
    reset settings
    reset token
    get initial balances
    transfer to pool.seeds    ✔  Given transfered SEEDS: should have the correct HPOOL balances

    transfer HPOOL    ✔  Given transfered HPOOL: should have the correct balances
    ✔  Given transfered HPOOL: should have the correct balances

    payout Seeds    ✔  Given payout SEEDS: should have the correct balances
    ✔  Given payout SEEDS: should have the correct balances

    transfer more HPOOL    ✔  Given transfer more HPOOL: should have the correct balances
    ✔  Given transfer more HPOOL: should have the correct balances

    payout more Seeds    ✔  Given payout more SEEDS: should have the correct balances
    ✔  Given payout more SEEDS: should have the correct balances

    payout all the Seeds    ✔  Given payout all SEEDS: should have the correct balances
    ✔  Given payout all SEEDS: should have the correct balances

    get balances after    ✔  Given all the payouts completed: should have the correct Seeds balance

[chuck-h]

Merged into PR #439