add reward over time calculation to exodus address balance

* all_reward is 10% of the mastercoins sold during the bootstrap
* seconds_passed = last_block_timestamp - exodus_bootstrap_deadline
* years = seconds_passed / seconds_in_one_year
* part_available = 1 - 0.5 ** years
* available_reward = all_reward * part_available

The constants are:
exodus_bootstrap_deadline = 1377993600
seconds_in_one_year=31556926

This calculation is also done for the validation of each simple send
that exodus address sent (to verify that the amount does not exceed
the balance at that moment).