Create safe payment contarct

[anchupin]

Create contract to allow apps to use safe contract for payments

1. Contract should support tokens/coins refund within N time
2. Contract should block tokens/coins for period to prevent scam from app creators
3. Create checkpoints when user should sign transaction and won't be able to make a refund (for example user bought a game and after 2nd level of game it won't be possible to make a refund because much content was consumed)