Proposal: Ability to exchange between any token pair in a single Exchange

[gundas]

As I've wrote in my issue #19, for the Liquidity Providers in makes economical sense to add liquidity only if the price ratio of UniSwap contract assets' is more or less stable. For example a pair DAI <-> GUSD would make sense - the liquidity providers would not risk their funds too much, but they could still collect the fees.

Having two exchanges ETH <-> DAI and ETH <-> GUSD does not fix the price fluctuation problem for the liquidity providers, since they will still loose funds when Ether price changes.

I think this is the only alternative to implementing Oracle to update the prices - UniSwap could work very well in trading assets which relative price does not change much.

P.s. I am not sure how the Ether case should be handled in code - it must either have separate handling logic (more complexity) or it must be wrapped to WETH (extra gas).