Update arbitrage_trader.py

Update:The 'compare' script is more efficient. Each price is fetched only once so you can run more often without the risk your IP address is blocked. Further after an error the trader prints a message and continues to run on the other exchanges. Fixed a bug on Coins-E 'GetDepth'.
Downgrade: looping over different coins has been removed from the 'Compare' function, but the next update can realise it easily in the 'Main'.