addLiquidity should not always consume the full token amounts

[0xduality]

Currently addLiquidity receives the full amounts of tokens specified in the arguments and mints an amount of liquidity determined by the minimum of the two tokens. This is problematic because it's impossible to know the exact amount needed for each token before the transaction is executed.

A better implementation would first determine the liquidity that would be minted, then the amount of tokens implied, then transfer those amounts in.