Considerations on Pool Creation and Swap with better overall understanding of the Exchange contract

[pedronauck]

• We should add slippage on swap and pool this creates a tolerance if the reserves change during the time user hits swap and it goes through the contract
• To add liquidity to a pool users need to provide the same ratio as the existing pool

[luizstacio]

open issues: #144, #145, #146, #147, #148, #149, #150