base58.hs haskoiner reading

Basically, now the en(de)coding to base58 is done by readInt and showIntAtBase from Numeric. Im not sure if its faster but is easier to read. Moreover I created the function decodeBase58I to be consistent with having all the encoding functions and its inverse.  
If you make this function take Integral a instead of Integer, you do not need the fromIntegral in this line.