profitability calculation added; bugfix of multiple data download

CryptoSwitcher can now calculate the profitabilty by itself by using the current difficulty and market sell prices. The difficulty values are downloaded from CoinChoose, except for TerraCoin, due to its high fluctuation, a more accurate difficulty is downloaded from cryptocoinexplorer.com. Prices are downloaded from the markets selected in the config. Currently available options are Bter, BTC-E and Virutex. More markets are added, as soon as they have a JSON API.
Also see config sample for more details.

A small bugfix was done when using coinchoose as data source. In the previous versions the get the profitability, a web site download was done for every coin. Since one download already contains all the necessary data, any further download just caused the calculation to be slower and also increased the server load.